diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 3696506eb3..631d5c7d15 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -64,6 +64,12 @@ The file given at `<path>` is expected to be readable, and can contain
 duplicates. (If a given OID is given more than once, it is marked as
 preferred if at least one instance of it begins with the special `+`
 marker).
+
+	--incremental::
+		Write an incremental MIDX file containing only objects
+		and packs not present in an existing MIDX layer.
+		Migrates non-incremental MIDXs to incremental ones when
+		necessary. Incompatible with `--bitmap`.
 --
 
 verify::
@@ -74,6 +80,8 @@ expire::
 	have no objects referenced by the MIDX (with the exception of
 	`.keep` packs and cruft packs). Rewrite the MIDX file afterward
 	to remove all references to these pack-files.
++
+NOTE: this mode is incompatible with incremental MIDX files.
 
 repack::
 	Create a new pack-file containing objects in small pack-files
@@ -95,7 +103,8 @@ repack::
 +
 If `repack.packKeptObjects` is `false`, then any pack-files with an
 associated `.keep` file will not be selected for the batch to repack.
-
++
+NOTE: this mode is incompatible with incremental MIDX files.
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 9cf1a32d65..8805cbbeb3 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -129,6 +129,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv,
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BIT(0, "progress", &opts.flags,
 			N_("force progress reporting"), MIDX_PROGRESS),
+		OPT_BIT(0, "incremental", &opts.flags,
+			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
 		OPT_BOOL(0, "stdin-packs", &opts.stdin_packs,
 			 N_("write multi-pack index containing only given indexes")),
 		OPT_FILENAME(0, "refs-snapshot", &opts.refs_snapshot,
diff --git a/builtin/repack.c b/builtin/repack.c
index 8499bf0e12..7608430a37 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -1514,8 +1514,12 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	if (run_update_server_info)
 		update_server_info(0);
 
-	if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX, 0))
-		write_midx_file(get_object_directory(), NULL, NULL, 0);
+	if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX, 0)) {
+		unsigned flags = 0;
+		if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL, 0))
+			flags |= MIDX_WRITE_INCREMENTAL;
+		write_midx_file(get_object_directory(), NULL, NULL, flags);
+	}
 
 cleanup:
 	string_list_clear(&names, 1);
diff --git a/ci/run-build-and-tests.sh b/ci/run-build-and-tests.sh
index e6fd68630c..2e28d02b20 100755
--- a/ci/run-build-and-tests.sh
+++ b/ci/run-build-and-tests.sh
@@ -25,6 +25,7 @@ linux-TEST-vars)
 	export GIT_TEST_COMMIT_GRAPH=1
 	export GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS=1
 	export GIT_TEST_MULTI_PACK_INDEX=1
+	export GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=1
 	export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master
 	export GIT_TEST_NO_WRITE_REV_INDEX=1
 	export GIT_TEST_CHECKOUT_WORKERS=2
diff --git a/midx-write.c b/midx-write.c
index d5275d719b..a94cb28bfd 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -17,6 +17,8 @@
 #include "refs.h"
 #include "revision.h"
 #include "list-objects.h"
+#include "path.h"
+#include "pack-revindex.h"
 
 #define PACK_EXPIRED UINT_MAX
 #define BITMAP_POS_UNKNOWN (~((uint32_t)0))
@@ -25,7 +27,11 @@
 
 extern int midx_checksum_valid(struct multi_pack_index *m);
 extern void clear_midx_files_ext(const char *object_dir, const char *ext,
-				 unsigned char *keep_hash);
+				 const char *keep_hash);
+extern void clear_incremental_midx_files_ext(const char *object_dir,
+					     const char *ext,
+					     const char **keep_hashes,
+					     uint32_t hashes_nr);
 extern int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 				const char *idx_name);
 
@@ -86,6 +92,7 @@ struct write_midx_context {
 	size_t nr;
 	size_t alloc;
 	struct multi_pack_index *m;
+	struct multi_pack_index *base_midx;
 	struct progress *progress;
 	unsigned pack_paths_checked;
 
@@ -99,6 +106,9 @@ struct write_midx_context {
 
 	int preferred_pack_idx;
 
+	int incremental;
+	uint32_t num_multi_pack_indexes_before;
+
 	struct string_list *to_include;
 };
 
@@ -122,6 +132,9 @@ static int should_include_pack(const struct write_midx_context *ctx,
 	 */
 	if (ctx->m && midx_contains_pack(ctx->m, file_name))
 		return 0;
+	else if (ctx->base_midx && midx_contains_pack(ctx->base_midx,
+						      file_name))
+		return 0;
 	else if (ctx->to_include &&
 		 !string_list_has_string(ctx->to_include, file_name))
 		return 0;
@@ -338,7 +351,7 @@ static void compute_sorted_entries(struct write_midx_context *ctx,
 	for (cur_fanout = 0; cur_fanout < 256; cur_fanout++) {
 		fanout.nr = 0;
 
-		if (ctx->m)
+		if (ctx->m && !ctx->incremental)
 			midx_fanout_add_midx_fanout(&fanout, ctx->m, cur_fanout,
 						    ctx->preferred_pack_idx);
 
@@ -364,6 +377,10 @@ static void compute_sorted_entries(struct write_midx_context *ctx,
 			if (cur_object && oideq(&fanout.entries[cur_object - 1].oid,
 						&fanout.entries[cur_object].oid))
 				continue;
+			if (ctx->incremental && ctx->base_midx &&
+			    midx_has_oid(ctx->base_midx,
+					 &fanout.entries[cur_object].oid))
+				continue;
 
 			ALLOC_GROW(ctx->entries, st_add(ctx->entries_nr, 1),
 				   alloc_objects);
@@ -547,10 +564,16 @@ static int write_midx_revindex(struct hashfile *f,
 			       void *data)
 {
 	struct write_midx_context *ctx = data;
-	uint32_t i;
+	uint32_t i, nr_base;
+
+	if (ctx->incremental && ctx->base_midx)
+		nr_base = ctx->base_midx->num_objects +
+			ctx->base_midx->num_objects_in_base;
+	else
+		nr_base = 0;
 
 	for (i = 0; i < ctx->entries_nr; i++)
-		hashwrite_be32(f, ctx->pack_order[i]);
+		hashwrite_be32(f, ctx->pack_order[i] + nr_base);
 
 	return 0;
 }
@@ -579,12 +602,18 @@ static int midx_pack_order_cmp(const void *va, const void *vb)
 static uint32_t *midx_pack_order(struct write_midx_context *ctx)
 {
 	struct midx_pack_order_data *data;
-	uint32_t *pack_order;
+	uint32_t *pack_order, base_objects = 0;
 	uint32_t i;
 
 	trace2_region_enter("midx", "midx_pack_order", the_repository);
 
+	if (ctx->incremental && ctx->base_midx)
+		base_objects = ctx->base_midx->num_objects +
+			ctx->base_midx->num_objects_in_base;
+
+	ALLOC_ARRAY(pack_order, ctx->entries_nr);
 	ALLOC_ARRAY(data, ctx->entries_nr);
+
 	for (i = 0; i < ctx->entries_nr; i++) {
 		struct pack_midx_entry *e = &ctx->entries[i];
 		data[i].nr = i;
@@ -596,12 +625,11 @@ static uint32_t *midx_pack_order(struct write_midx_context *ctx)
 
 	QSORT(data, ctx->entries_nr, midx_pack_order_cmp);
 
-	ALLOC_ARRAY(pack_order, ctx->entries_nr);
 	for (i = 0; i < ctx->entries_nr; i++) {
 		struct pack_midx_entry *e = &ctx->entries[data[i].nr];
 		struct pack_info *pack = &ctx->info[ctx->pack_perm[e->pack_int_id]];
 		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
-			pack->bitmap_pos = i;
+			pack->bitmap_pos = i + base_objects;
 		pack->bitmap_nr++;
 		pack_order[i] = data[i].nr;
 	}
@@ -649,7 +677,8 @@ static void prepare_midx_packing_data(struct packing_data *pdata,
 	prepare_packing_data(the_repository, pdata);
 
 	for (i = 0; i < ctx->entries_nr; i++) {
-		struct pack_midx_entry *from = &ctx->entries[ctx->pack_order[i]];
+		uint32_t pos = ctx->pack_order[i];
+		struct pack_midx_entry *from = &ctx->entries[pos];
 		struct object_entry *to = packlist_alloc(pdata, &from->oid);
 
 		oe_set_in_pack(pdata, to,
@@ -897,35 +926,128 @@ cleanup:
 static int fill_packs_from_midx(struct write_midx_context *ctx,
 				const char *preferred_pack_name, uint32_t flags)
 {
-	uint32_t i;
+	struct multi_pack_index *m;
 
-	for (i = 0; i < ctx->m->num_packs; i++) {
-		ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
+	for (m = ctx->m; m; m = m->base_midx) {
+		uint32_t i;
+
+		for (i = 0; i < m->num_packs; i++) {
+			ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
 
-		if (flags & MIDX_WRITE_REV_INDEX || preferred_pack_name) {
 			/*
 			 * If generating a reverse index, need to have
 			 * packed_git's loaded to compare their
 			 * mtimes and object count.
 			 *
-			 *
 			 * If a preferred pack is specified, need to
 			 * have packed_git's loaded to ensure the chosen
 			 * preferred pack has a non-zero object count.
 			 */
-			if (prepare_midx_pack(the_repository, ctx->m, i))
-				return error(_("could not load pack"));
+			if (flags & MIDX_WRITE_REV_INDEX ||
+			    preferred_pack_name) {
+				if (prepare_midx_pack(the_repository, m,
+						      m->num_packs_in_base + i)) {
+					error(_("could not load pack"));
+					return 1;
+				}
 
-			if (open_pack_index(ctx->m->packs[i]))
-				die(_("could not open index for %s"),
-				    ctx->m->packs[i]->pack_name);
+				if (open_pack_index(m->packs[i]))
+					die(_("could not open index for %s"),
+					    m->packs[i]->pack_name);
+			}
+
+			fill_pack_info(&ctx->info[ctx->nr++], m->packs[i],
+				       m->pack_names[i],
+				       m->num_packs_in_base + i);
 		}
+	}
+	return 0;
+}
 
-		fill_pack_info(&ctx->info[ctx->nr++], ctx->m->packs[i],
-			       ctx->m->pack_names[i], i);
+static struct {
+	const char *non_split;
+	const char *split;
+} midx_exts[] = {
+	{NULL, MIDX_EXT_MIDX},
+	{MIDX_EXT_BITMAP, MIDX_EXT_BITMAP},
+	{MIDX_EXT_REV, MIDX_EXT_REV},
+};
+
+static int link_midx_to_chain(struct multi_pack_index *m)
+{
+	struct strbuf from = STRBUF_INIT;
+	struct strbuf to = STRBUF_INIT;
+	int ret = 0;
+	size_t i;
+
+	if (!m || m->has_chain) {
+		/*
+		 * Either no MIDX previously existed, or it was already
+		 * part of a MIDX chain. In both cases, we have nothing
+		 * to link, so return early.
+		 */
+		goto done;
 	}
 
-	return 0;
+	for (i = 0; i < ARRAY_SIZE(midx_exts); i++) {
+		const unsigned char *hash = get_midx_checksum(m);
+
+		get_midx_filename_ext(&from, m->object_dir, hash,
+				      midx_exts[i].non_split);
+		get_split_midx_filename_ext(&to, m->object_dir, hash,
+					    midx_exts[i].split);
+
+		if (link(from.buf, to.buf) < 0 && errno != ENOENT) {
+			ret = error_errno(_("unable to link '%s' to '%s'"),
+					  from.buf, to.buf);
+			goto done;
+		}
+
+		strbuf_reset(&from);
+		strbuf_reset(&to);
+	}
+
+done:
+	strbuf_release(&from);
+	strbuf_release(&to);
+	return ret;
+}
+
+static void clear_midx_files(const char *object_dir,
+			     const char **hashes,
+			     uint32_t hashes_nr,
+			     unsigned incremental)
+{
+	/*
+	 * if incremental:
+	 *   - remove all non-incremental MIDX files
+	 *   - remove any incremental MIDX files not in the current one
+	 *
+	 * if non-incremental:
+	 *   - remove all incremental MIDX files
+	 *   - remove any non-incremental MIDX files not matching the current
+	 *     hash
+	 */
+	struct strbuf buf = STRBUF_INIT;
+	const char *exts[] = { MIDX_EXT_BITMAP, MIDX_EXT_REV, MIDX_EXT_MIDX };
+	uint32_t i, j;
+
+	for (i = 0; i < ARRAY_SIZE(exts); i++) {
+		clear_incremental_midx_files_ext(object_dir, exts[i],
+						 hashes, hashes_nr);
+		for (j = 0; j < hashes_nr; j++)
+			clear_midx_files_ext(object_dir, exts[i], hashes[j]);
+	}
+
+	if (incremental)
+		get_midx_filename(&buf, object_dir);
+	else
+		get_midx_chain_filename(&buf, object_dir);
+
+	if (unlink(buf.buf) && errno != ENOENT)
+		die_errno(_("failed to clear multi-pack-index at %s"), buf.buf);
+
+	strbuf_release(&buf);
 }
 
 static int write_midx_internal(const char *object_dir,
@@ -940,42 +1062,66 @@ static int write_midx_internal(const char *object_dir,
 	uint32_t i, start_pack;
 	struct hashfile *f = NULL;
 	struct lock_file lk;
+	struct tempfile *incr;
 	struct write_midx_context ctx = { 0 };
 	int bitmapped_packs_concat_len = 0;
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
 	int result = 0;
+	const char **keep_hashes = NULL;
 	struct chunkfile *cf;
 
 	trace2_region_enter("midx", "write_midx_internal", the_repository);
 
-	get_midx_filename(&midx_name, object_dir);
+	ctx.incremental = !!(flags & MIDX_WRITE_INCREMENTAL);
+	if (ctx.incremental && (flags & MIDX_WRITE_BITMAP))
+		die(_("cannot write incremental MIDX with bitmap"));
+
+	if (ctx.incremental)
+		strbuf_addf(&midx_name,
+			    "%s/pack/multi-pack-index.d/tmp_midx_XXXXXX",
+			    object_dir);
+	else
+		get_midx_filename(&midx_name, object_dir);
 	if (safe_create_leading_directories(midx_name.buf))
 		die_errno(_("unable to create leading directories of %s"),
 			  midx_name.buf);
 
-	if (!packs_to_include) {
-		/*
-		 * Only reference an existing MIDX when not filtering which
-		 * packs to include, since all packs and objects are copied
-		 * blindly from an existing MIDX if one is present.
-		 */
-		ctx.m = lookup_multi_pack_index(the_repository, object_dir);
-	}
+	if (!packs_to_include || ctx.incremental) {
+		struct multi_pack_index *m = lookup_multi_pack_index(the_repository,
+								     object_dir);
+		if (m && !midx_checksum_valid(m)) {
+			warning(_("ignoring existing multi-pack-index; checksum mismatch"));
+			m = NULL;
+		}
 
-	if (ctx.m && !midx_checksum_valid(ctx.m)) {
-		warning(_("ignoring existing multi-pack-index; checksum mismatch"));
-		ctx.m = NULL;
+		if (m) {
+			/*
+			 * Only reference an existing MIDX when not filtering
+			 * which packs to include, since all packs and objects
+			 * are copied blindly from an existing MIDX if one is
+			 * present.
+			 */
+			if (ctx.incremental)
+				ctx.base_midx = m;
+			else if (!packs_to_include)
+				ctx.m = m;
+		}
 	}
 
 	ctx.nr = 0;
-	ctx.alloc = ctx.m ? ctx.m->num_packs : 16;
+	ctx.alloc = ctx.m ? ctx.m->num_packs + ctx.m->num_packs_in_base : 16;
 	ctx.info = NULL;
 	ALLOC_ARRAY(ctx.info, ctx.alloc);
 
-	if (ctx.m && fill_packs_from_midx(&ctx, preferred_pack_name,
-					  flags) < 0) {
-		result = 1;
+	if (ctx.incremental) {
+		struct multi_pack_index *m = ctx.base_midx;
+		while (m) {
+			ctx.num_multi_pack_indexes_before++;
+			m = m->base_midx;
+		}
+	} else if (ctx.m && fill_packs_from_midx(&ctx, preferred_pack_name,
+						 flags) < 0) {
 		goto cleanup;
 	}
 
@@ -992,7 +1138,8 @@ static int write_midx_internal(const char *object_dir,
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
 	stop_progress(&ctx.progress);
 
-	if ((ctx.m && ctx.nr == ctx.m->num_packs) &&
+	if ((ctx.m && ctx.nr == ctx.m->num_packs + ctx.m->num_packs_in_base) &&
+	    !ctx.incremental &&
 	    !(packs_to_include || packs_to_drop)) {
 		struct bitmap_index *bitmap_git;
 		int bitmap_exists;
@@ -1008,12 +1155,14 @@ static int write_midx_internal(const char *object_dir,
 			 * corresponding bitmap (or one wasn't requested).
 			 */
 			if (!want_bitmap)
-				clear_midx_files_ext(object_dir, ".bitmap",
-						     NULL);
+				clear_midx_files_ext(object_dir, "bitmap", NULL);
 			goto cleanup;
 		}
 	}
 
+	if (ctx.incremental && !ctx.nr)
+		goto cleanup; /* nothing to do */
+
 	if (preferred_pack_name) {
 		ctx.preferred_pack_idx = -1;
 
@@ -1159,8 +1308,30 @@ static int write_midx_internal(const char *object_dir,
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
 					(pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
 
-	hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR);
-	f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
+	if (ctx.incremental) {
+		struct strbuf lock_name = STRBUF_INIT;
+
+		get_midx_chain_filename(&lock_name, object_dir);
+		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR);
+		strbuf_release(&lock_name);
+
+		incr = mks_tempfile_m(midx_name.buf, 0444);
+		if (!incr) {
+			error(_("unable to create temporary MIDX layer"));
+			return -1;
+		}
+
+		if (adjust_shared_perm(get_tempfile_path(incr))) {
+			error(_("unable to adjust shared permissions for '%s'"),
+			      get_tempfile_path(incr));
+			return -1;
+		}
+
+		f = hashfd(get_tempfile_fd(incr), get_tempfile_path(incr));
+	} else {
+		hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR);
+		f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
+	}
 
 	if (ctx.nr - dropped_packs == 0) {
 		error(_("no pack files to index."));
@@ -1253,14 +1424,55 @@ static int write_midx_internal(const char *object_dir,
 	 * have been freed in the previous if block.
 	 */
 
-	if (ctx.m)
+	CALLOC_ARRAY(keep_hashes, ctx.num_multi_pack_indexes_before + 1);
+
+	if (ctx.incremental) {
+		FILE *chainf = fdopen_lock_file(&lk, "w");
+		struct strbuf final_midx_name = STRBUF_INIT;
+		struct multi_pack_index *m = ctx.base_midx;
+
+		if (!chainf) {
+			error_errno(_("unable to open multi-pack-index chain file"));
+			return -1;
+		}
+
+		if (link_midx_to_chain(ctx.base_midx) < 0)
+			return -1;
+
+		get_split_midx_filename_ext(&final_midx_name, object_dir,
+					    midx_hash, MIDX_EXT_MIDX);
+
+		if (rename_tempfile(&incr, final_midx_name.buf) < 0) {
+			error_errno(_("unable to rename new multi-pack-index layer"));
+			return -1;
+		}
+
+		keep_hashes[ctx.num_multi_pack_indexes_before] =
+			xstrdup(hash_to_hex(midx_hash));
+
+		for (i = 0; i < ctx.num_multi_pack_indexes_before; i++) {
+			uint32_t j = ctx.num_multi_pack_indexes_before - i - 1;
+
+			keep_hashes[j] = xstrdup(hash_to_hex(get_midx_checksum(m)));
+			m = m->base_midx;
+		}
+
+		for (i = 0; i < ctx.num_multi_pack_indexes_before + 1; i++)
+			fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
+	} else {
+		keep_hashes[ctx.num_multi_pack_indexes_before] =
+			xstrdup(hash_to_hex(midx_hash));
+	}
+
+	if (ctx.m || ctx.base_midx)
 		close_object_store(the_repository->objects);
 
 	if (commit_lock_file(&lk) < 0)
 		die_errno(_("could not write multi-pack-index"));
 
-	clear_midx_files_ext(object_dir, ".bitmap", midx_hash);
-	clear_midx_files_ext(object_dir, ".rev", midx_hash);
+	clear_midx_files(object_dir, keep_hashes,
+			 ctx.num_multi_pack_indexes_before + 1,
+			 ctx.incremental);
 
 cleanup:
 	for (i = 0; i < ctx.nr; i++) {
@@ -1275,6 +1487,11 @@ cleanup:
 	free(ctx.entries);
 	free(ctx.pack_perm);
 	free(ctx.pack_order);
+	if (keep_hashes) {
+		for (i = 0; i < ctx.num_multi_pack_indexes_before + 1; i++)
+			free((char *)keep_hashes[i]);
+		free(keep_hashes);
+	}
 	strbuf_release(&midx_name);
 
 	trace2_region_leave("midx", "write_midx_internal", the_repository);
@@ -1311,6 +1528,9 @@ int expire_midx_packs(struct repository *r, const char *object_dir, unsigned fla
 	if (!m)
 		return 0;
 
+	if (m->base_midx)
+		die(_("cannot expire packs from an incremental multi-pack-index"));
+
 	CALLOC_ARRAY(count, m->num_packs);
 
 	if (flags & MIDX_PROGRESS)
@@ -1485,6 +1705,8 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
 
 	if (!m)
 		return 0;
+	if (m->base_midx)
+		die(_("cannot repack an incremental multi-pack-index"));
 
 	CALLOC_ARRAY(include_pack, m->num_packs);
 
diff --git a/midx.c b/midx.c
index a53d65702d..c867b2b6c2 100644
--- a/midx.c
+++ b/midx.c
@@ -16,7 +16,10 @@
 
 int midx_checksum_valid(struct multi_pack_index *m);
 void clear_midx_files_ext(const char *object_dir, const char *ext,
-			  unsigned char *keep_hash);
+			  const char *keep_hash);
+void clear_incremental_midx_files_ext(const char *object_dir, const char *ext,
+				      char **keep_hashes,
+				      uint32_t hashes_nr);
 int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 			 const char *idx_name);
 
@@ -521,6 +524,11 @@ int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m,
 	return 0;
 }
 
+int midx_has_oid(struct multi_pack_index *m, const struct object_id *oid)
+{
+	return bsearch_midx(oid, m, NULL);
+}
+
 struct object_id *nth_midxed_object_oid(struct object_id *oid,
 					struct multi_pack_index *m,
 					uint32_t n)
@@ -723,7 +731,8 @@ int midx_checksum_valid(struct multi_pack_index *m)
 }
 
 struct clear_midx_data {
-	char *keep;
+	char **keep;
+	uint32_t keep_nr;
 	const char *ext;
 };
 
@@ -731,32 +740,63 @@ static void clear_midx_file_ext(const char *full_path, size_t full_path_len UNUS
 				const char *file_name, void *_data)
 {
 	struct clear_midx_data *data = _data;
+	uint32_t i;
 
 	if (!(starts_with(file_name, "multi-pack-index-") &&
 	      ends_with(file_name, data->ext)))
 		return;
-	if (data->keep && !strcmp(data->keep, file_name))
-		return;
-
+	for (i = 0; i < data->keep_nr; i++) {
+		if (!strcmp(data->keep[i], file_name))
+			return;
+	}
 	if (unlink(full_path))
 		die_errno(_("failed to remove %s"), full_path);
 }
 
 void clear_midx_files_ext(const char *object_dir, const char *ext,
-			  unsigned char *keep_hash)
+			  const char *keep_hash)
 {
 	struct clear_midx_data data;
 	memset(&data, 0, sizeof(struct clear_midx_data));
 
-	if (keep_hash)
-		data.keep = xstrfmt("multi-pack-index-%s%s",
-				    hash_to_hex(keep_hash), ext);
+	if (keep_hash) {
+		ALLOC_ARRAY(data.keep, 1);
+
+		data.keep[0] = xstrfmt("multi-pack-index-%s.%s", keep_hash, ext);
+		data.keep_nr = 1;
+	}
 	data.ext = ext;
 
 	for_each_file_in_pack_dir(object_dir,
 				  clear_midx_file_ext,
 				  &data);
 
+	if (keep_hash)
+		free(data.keep[0]);
+	free(data.keep);
+}
+
+void clear_incremental_midx_files_ext(const char *object_dir, const char *ext,
+				      char **keep_hashes,
+				      uint32_t hashes_nr)
+{
+	struct clear_midx_data data;
+	uint32_t i;
+
+	memset(&data, 0, sizeof(struct clear_midx_data));
+
+	ALLOC_ARRAY(data.keep, hashes_nr);
+	for (i = 0; i < hashes_nr; i++)
+		data.keep[i] = xstrfmt("multi-pack-index-%s.%s", keep_hashes[i],
+				       ext);
+	data.keep_nr = hashes_nr;
+	data.ext = ext;
+
+	for_each_file_in_pack_subdir(object_dir, "multi-pack-index.d",
+				     clear_midx_file_ext, &data);
+
+	for (i = 0; i < hashes_nr; i++)
+		free(data.keep[i]);
 	free(data.keep);
 }
 
@@ -774,8 +814,8 @@ void clear_midx_file(struct repository *r)
 	if (remove_path(midx.buf))
 		die(_("failed to clear multi-pack-index at %s"), midx.buf);
 
-	clear_midx_files_ext(r->objects->odb->path, ".bitmap", NULL);
-	clear_midx_files_ext(r->objects->odb->path, ".rev", NULL);
+	clear_midx_files_ext(r->objects->odb->path, MIDX_EXT_BITMAP, NULL);
+	clear_midx_files_ext(r->objects->odb->path, MIDX_EXT_REV, NULL);
 
 	strbuf_release(&midx);
 }
diff --git a/midx.h b/midx.h
index 3714cad2cc..42d4f8d149 100644
--- a/midx.h
+++ b/midx.h
@@ -29,6 +29,8 @@ struct bitmapped_pack;
 #define MIDX_LARGE_OFFSET_NEEDED 0x80000000
 
 #define GIT_TEST_MULTI_PACK_INDEX "GIT_TEST_MULTI_PACK_INDEX"
+#define GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL \
+	"GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL"
 
 struct multi_pack_index {
 	struct multi_pack_index *next;
@@ -77,6 +79,7 @@ struct multi_pack_index {
 #define MIDX_WRITE_BITMAP (1 << 2)
 #define MIDX_WRITE_BITMAP_HASH_CACHE (1 << 3)
 #define MIDX_WRITE_BITMAP_LOOKUP_TABLE (1 << 4)
+#define MIDX_WRITE_INCREMENTAL (1 << 5)
 
 #define MIDX_EXT_REV "rev"
 #define MIDX_EXT_BITMAP "bitmap"
@@ -101,6 +104,7 @@ int bsearch_one_midx(const struct object_id *oid, struct multi_pack_index *m,
 		     uint32_t *result);
 int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m,
 		 uint32_t *result);
+int midx_has_oid(struct multi_pack_index *m, const struct object_id *oid);
 off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos);
 uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos);
 struct object_id *nth_midxed_object_oid(struct object_id *oid,
diff --git a/packfile.c b/packfile.c
index 1eb18e3041..cf12a539ea 100644
--- a/packfile.c
+++ b/packfile.c
@@ -815,9 +815,10 @@ static void report_pack_garbage(struct string_list *list)
 	report_helper(list, seen_bits, first, list->nr);
 }
 
-void for_each_file_in_pack_dir(const char *objdir,
-			       each_file_in_pack_dir_fn fn,
-			       void *data)
+void for_each_file_in_pack_subdir(const char *objdir,
+				  const char *subdir,
+				  each_file_in_pack_dir_fn fn,
+				  void *data)
 {
 	struct strbuf path = STRBUF_INIT;
 	size_t dirnamelen;
@@ -826,6 +827,8 @@ void for_each_file_in_pack_dir(const char *objdir,
 
 	strbuf_addstr(&path, objdir);
 	strbuf_addstr(&path, "/pack");
+	if (subdir)
+		strbuf_addf(&path, "/%s", subdir);
 	dir = opendir(path.buf);
 	if (!dir) {
 		if (errno != ENOENT)
@@ -847,6 +850,13 @@ void for_each_file_in_pack_dir(const char *objdir,
 	strbuf_release(&path);
 }
 
+void for_each_file_in_pack_dir(const char *objdir,
+			       each_file_in_pack_dir_fn fn,
+			       void *data)
+{
+	for_each_file_in_pack_subdir(objdir, NULL, fn, data);
+}
+
 struct prepare_pack_data {
 	struct repository *r;
 	struct string_list *garbage;
diff --git a/packfile.h b/packfile.h
index eb18ec15db..0f78658229 100644
--- a/packfile.h
+++ b/packfile.h
@@ -55,6 +55,10 @@ struct packed_git *parse_pack_index(unsigned char *sha1, const char *idx_path);
 
 typedef void each_file_in_pack_dir_fn(const char *full_path, size_t full_path_len,
 				      const char *file_name, void *data);
+void for_each_file_in_pack_subdir(const char *objdir,
+				  const char *subdir,
+				  each_file_in_pack_dir_fn fn,
+				  void *data);
 void for_each_file_in_pack_dir(const char *objdir,
 			       each_file_in_pack_dir_fn fn,
 			       void *data);
diff --git a/t/README b/t/README
index e8a11926e4..e93a29de1b 100644
--- a/t/README
+++ b/t/README
@@ -469,6 +469,10 @@ GIT_TEST_MULTI_PACK_INDEX=<boolean>, when true, forces the multi-pack-
 index to be written after every 'git repack' command, and overrides the
 'core.multiPackIndex' setting to true.
 
+GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=<boolean>, when true, sets
+the '--incremental' option on all invocations of 'git multi-pack-index
+write'.
+
 GIT_TEST_SIDEBAND_ALL=<boolean>, when true, overrides the
 'uploadpack.allowSidebandAll' setting to true, and when false, forces
 fetch-pack to not request sideband-all (even if the server advertises
diff --git a/t/lib-bitmap.sh b/t/lib-bitmap.sh
index f595937094..62aa6744a6 100644
--- a/t/lib-bitmap.sh
+++ b/t/lib-bitmap.sh
@@ -1,6 +1,8 @@
 # Helpers for scripts testing bitmap functionality; see t5310 for
 # example usage.
 
+. "$TEST_DIRECTORY"/lib-midx.sh
+
 objdir=.git/objects
 midx=$objdir/pack/multi-pack-index
 
@@ -264,10 +266,6 @@ have_delta () {
 	test_cmp expect actual
 }
 
-midx_checksum () {
-	test-tool read-midx --checksum "$1"
-}
-
 # midx_pack_source <obj>
 midx_pack_source () {
 	test-tool read-midx --show-objects .git/objects | grep "^$1 " | cut -f2
diff --git a/t/lib-midx.sh b/t/lib-midx.sh
index 1261994744..e38c609604 100644
--- a/t/lib-midx.sh
+++ b/t/lib-midx.sh
@@ -6,3 +6,31 @@ test_midx_consistent () {
 	test_cmp expect actual &&
 	git multi-pack-index --object-dir=$1 verify
 }
+
+midx_checksum () {
+	test-tool read-midx --checksum "$1"
+}
+
+midx_git_two_modes () {
+	git -c core.multiPackIndex=false $1 >expect &&
+	git -c core.multiPackIndex=true $1 >actual &&
+	if [ "$2" = "sorted" ]
+	then
+		sort <expect >expect.sorted &&
+		mv expect.sorted expect &&
+		sort <actual >actual.sorted &&
+		mv actual.sorted actual
+	fi &&
+	test_cmp expect actual
+}
+
+compare_results_with_midx () {
+	MSG=$1
+	test_expect_success "check normal git operations: $MSG" '
+		midx_git_two_modes "rev-list --objects --all" &&
+		midx_git_two_modes "log --raw" &&
+		midx_git_two_modes "count-objects --verbose" &&
+		midx_git_two_modes "cat-file --batch-all-objects --batch-check" &&
+		midx_git_two_modes "cat-file --batch-all-objects --batch-check --unordered" sorted
+	'
+}
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 6e9ee23398..4b0b5a5c9f 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -3,8 +3,11 @@
 test_description='multi-pack-indexes'
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-chunk.sh
+. "$TEST_DIRECTORY"/lib-midx.sh
 
 GIT_TEST_MULTI_PACK_INDEX=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=0
 objdir=.git/objects
 
 HASH_LEN=$(test_oid rawsz)
@@ -107,30 +110,6 @@ test_expect_success 'write midx with one v1 pack' '
 	midx_read_expect 1 18 4 $objdir
 '
 
-midx_git_two_modes () {
-	git -c core.multiPackIndex=false $1 >expect &&
-	git -c core.multiPackIndex=true $1 >actual &&
-	if [ "$2" = "sorted" ]
-	then
-		sort <expect >expect.sorted &&
-		mv expect.sorted expect &&
-		sort <actual >actual.sorted &&
-		mv actual.sorted actual
-	fi &&
-	test_cmp expect actual
-}
-
-compare_results_with_midx () {
-	MSG=$1
-	test_expect_success "check normal git operations: $MSG" '
-		midx_git_two_modes "rev-list --objects --all" &&
-		midx_git_two_modes "log --raw" &&
-		midx_git_two_modes "count-objects --verbose" &&
-		midx_git_two_modes "cat-file --batch-all-objects --batch-check" &&
-		midx_git_two_modes "cat-file --batch-all-objects --batch-check --unordered" sorted
-	'
-}
-
 test_expect_success 'write midx with one v2 pack' '
 	git pack-objects --index-version=2,0x40 $objdir/pack/test <obj-list &&
 	git multi-pack-index --object-dir=$objdir write &&
diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh
index 1cb3e3ff08..832b92619c 100755
--- a/t/t5326-multi-pack-bitmaps.sh
+++ b/t/t5326-multi-pack-bitmaps.sh
@@ -7,6 +7,7 @@ test_description='exercise basic multi-pack bitmap functionality'
 # We'll be writing our own MIDX, so avoid getting confused by the
 # automatic ones.
 GIT_TEST_MULTI_PACK_INDEX=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=0
 
 # This test exercise multi-pack bitmap functionality where the object order is
 # stored and read from a special chunk within the MIDX, so use the default
diff --git a/t/t5327-multi-pack-bitmaps-rev.sh b/t/t5327-multi-pack-bitmaps-rev.sh
index 23db949c20..9cac03a94b 100755
--- a/t/t5327-multi-pack-bitmaps-rev.sh
+++ b/t/t5327-multi-pack-bitmaps-rev.sh
@@ -8,6 +8,7 @@ test_description='exercise basic multi-pack bitmap functionality (.rev files)'
 # We'll be writing our own MIDX, so avoid getting confused by the automatic
 # ones.
 GIT_TEST_MULTI_PACK_INDEX=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=0
 
 # Unlike t5326, this test exercise multi-pack bitmap functionality where the
 # object order is stored in a separate .rev file.
diff --git a/t/t5332-multi-pack-reuse.sh b/t/t5332-multi-pack-reuse.sh
index ed823f37bc..941e73d354 100755
--- a/t/t5332-multi-pack-reuse.sh
+++ b/t/t5332-multi-pack-reuse.sh
@@ -6,6 +6,8 @@ TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-bitmap.sh
 
+GIT_TEST_MULTI_PACK_INDEX=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=0
 objdir=.git/objects
 packdir=$objdir/pack
 
diff --git a/t/t5334-incremental-multi-pack-index.sh b/t/t5334-incremental-multi-pack-index.sh
new file mode 100755
index 0000000000..c3b08acc73
--- /dev/null
+++ b/t/t5334-incremental-multi-pack-index.sh
@@ -0,0 +1,46 @@
+#!/bin/sh
+
+test_description='incremental multi-pack-index'
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-midx.sh
+
+GIT_TEST_MULTI_PACK_INDEX=0
+export GIT_TEST_MULTI_PACK_INDEX
+
+objdir=.git/objects
+packdir=$objdir/pack
+midxdir=$packdir/multi-pack-index.d
+midx_chain=$midxdir/multi-pack-index-chain
+
+test_expect_success 'convert non-incremental MIDX to incremental' '
+	test_commit base &&
+	git repack -ad &&
+	git multi-pack-index write &&
+
+	test_path_is_file $packdir/multi-pack-index &&
+	old_hash="$(midx_checksum $objdir)" &&
+
+	test_commit other &&
+	git repack -d &&
+	git multi-pack-index write --incremental &&
+
+	test_path_is_missing $packdir/multi-pack-index &&
+	test_path_is_file $midx_chain &&
+	test_line_count = 2 $midx_chain &&
+	grep $old_hash $midx_chain
+'
+
+compare_results_with_midx 'incremental MIDX'
+
+test_expect_success 'convert incremental to non-incremental' '
+	test_commit squash &&
+	git repack -d &&
+	git multi-pack-index write &&
+
+	test_path_is_file $packdir/multi-pack-index &&
+	test_dir_is_empty $midxdir
+'
+
+compare_results_with_midx 'non-incremental MIDX conversion'
+
+test_done
diff --git a/t/t7700-repack.sh b/t/t7700-repack.sh
index 8f34f05087..be1188e736 100755
--- a/t/t7700-repack.sh
+++ b/t/t7700-repack.sh
@@ -7,6 +7,9 @@ test_description='git repack works correctly'
 . "${TEST_DIRECTORY}/lib-midx.sh"
 . "${TEST_DIRECTORY}/lib-terminal.sh"
 
+GIT_TEST_MULTI_PACK_INDEX=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL=0
+
 commit_and_pack () {
 	test_commit "$@" 1>&2 &&
 	incrpackid=$(git pack-objects --all --unpacked --incremental .git/objects/pack/pack </dev/null) &&
@@ -117,7 +120,7 @@ test_expect_success '--local disables writing bitmaps when connected to alternat
 	(
 		cd member &&
 		test_commit "object" &&
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Adl --write-bitmap-index 2>err &&
+		git repack -Adl --write-bitmap-index 2>err &&
 		cat >expect <<-EOF &&
 		warning: disabling bitmap writing, as some objects are not being packed
 		EOF
@@ -533,11 +536,11 @@ test_expect_success 'setup for --write-midx tests' '
 test_expect_success '--write-midx unchanged' '
 	(
 		cd midx &&
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack &&
+		git repack &&
 		test_path_is_missing $midx &&
 		test_path_is_missing $midx-*.bitmap &&
 
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack --write-midx &&
+		git repack --write-midx &&
 
 		test_path_is_file $midx &&
 		test_path_is_missing $midx-*.bitmap &&
@@ -550,7 +553,7 @@ test_expect_success '--write-midx with a new pack' '
 		cd midx &&
 		test_commit loose &&
 
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack --write-midx &&
+		git repack --write-midx &&
 
 		test_path_is_file $midx &&
 		test_path_is_missing $midx-*.bitmap &&
@@ -561,7 +564,7 @@ test_expect_success '--write-midx with a new pack' '
 test_expect_success '--write-midx with -b' '
 	(
 		cd midx &&
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack -mb &&
+		git repack -mb &&
 
 		test_path_is_file $midx &&
 		test_path_is_file $midx-*.bitmap &&
@@ -574,7 +577,7 @@ test_expect_success '--write-midx with -d' '
 		cd midx &&
 		test_commit repack &&
 
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Ad --write-midx &&
+		git repack -Ad --write-midx &&
 
 		test_path_is_file $midx &&
 		test_path_is_missing $midx-*.bitmap &&
@@ -587,21 +590,21 @@ test_expect_success 'cleans up MIDX when appropriate' '
 		cd midx &&
 
 		test_commit repack-2 &&
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Adb --write-midx &&
+		git repack -Adb --write-midx &&
 
 		checksum=$(midx_checksum $objdir) &&
 		test_path_is_file $midx &&
 		test_path_is_file $midx-$checksum.bitmap &&
 
 		test_commit repack-3 &&
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Adb --write-midx &&
+		git repack -Adb --write-midx &&
 
 		test_path_is_file $midx &&
 		test_path_is_missing $midx-$checksum.bitmap &&
 		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
 
 		test_commit repack-4 &&
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Adb &&
+		git repack -Adb &&
 
 		find $objdir/pack -type f -name "multi-pack-index*" >files &&
 		test_must_be_empty files
@@ -622,7 +625,6 @@ test_expect_success '--write-midx with preferred bitmap tips' '
 		git log --format="create refs/tags/%s/%s %H" HEAD >refs &&
 		git update-ref --stdin <refs &&
 
-		GIT_TEST_MULTI_PACK_INDEX=0 \
 		git repack --write-midx --write-bitmap-index &&
 		test_path_is_file $midx &&
 		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
@@ -714,13 +716,13 @@ test_expect_success '--write-midx removes stale pack-based bitmaps' '
 	(
 		cd repo &&
 		test_commit base &&
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Ab &&
+		git repack -Ab &&
 
 		pack_bitmap=$(ls $objdir/pack/pack-*.bitmap) &&
 		test_path_is_file "$pack_bitmap" &&
 
 		test_commit tip &&
-		GIT_TEST_MULTI_PACK_INDEX=0 git repack -bm &&
+		git repack -bm &&
 
 		test_path_is_file $midx &&
 		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
@@ -743,7 +745,6 @@ test_expect_success '--write-midx with --pack-kept-objects' '
 		keep="$objdir/pack/pack-$one.keep" &&
 		touch "$keep" &&
 
-		GIT_TEST_MULTI_PACK_INDEX=0 \
 		git repack --write-midx --write-bitmap-index --geometric=2 -d \
 			--pack-kept-objects &&