diff --git a/Documentation/git-merge-tree.txt b/Documentation/git-merge-tree.txt
index d6c356740ef..e762209b76d 100644
--- a/Documentation/git-merge-tree.txt
+++ b/Documentation/git-merge-tree.txt
@@ -64,6 +64,10 @@ OPTIONS
 	share no common history.  This flag can be given to override that
 	check and make the merge proceed anyway.
 
+--merge-base=<commit>::
+	Instead of finding the merge-bases for <branch1> and <branch2>,
+	specify a merge-base for the merge.
+
 [[OUTPUT]]
 OUTPUT
 ------
diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
index ae5782917b9..089ea8fac81 100644
--- a/builtin/merge-tree.c
+++ b/builtin/merge-tree.c
@@ -3,6 +3,7 @@
 #include "tree-walk.h"
 #include "xdiff-interface.h"
 #include "help.h"
+#include "commit.h"
 #include "commit-reach.h"
 #include "merge-ort.h"
 #include "object-store.h"
@@ -402,6 +403,7 @@ struct merge_tree_options {
 	int allow_unrelated_histories;
 	int show_messages;
 	int name_only;
+	const struct commit *base_commit;
 };
 
 static int real_merge(struct merge_tree_options *o,
@@ -430,16 +432,26 @@ static int real_merge(struct merge_tree_options *o,
 	opt.branch1 = branch1;
 	opt.branch2 = branch2;
 
-	/*
-	 * Get the merge bases, in reverse order; see comment above
-	 * merge_incore_recursive in merge-ort.h
-	 */
-	merge_bases = get_merge_bases(parent1, parent2);
-	if (!merge_bases && !o->allow_unrelated_histories)
-		die(_("refusing to merge unrelated histories"));
-	merge_bases = reverse_commit_list(merge_bases);
+	if (o->base_commit) {
+		struct tree *base_tree, *parent1_tree, *parent2_tree;
+
+		opt.ancestor = "specified merge base";
+		base_tree = get_commit_tree(o->base_commit);
+		parent1_tree = get_commit_tree(parent1);
+		parent2_tree = get_commit_tree(parent2);
+		merge_incore_nonrecursive(&opt, base_tree, parent1_tree, parent2_tree, &result);
+	} else {
+		/*
+		 * Get the merge bases, in reverse order; see comment above
+		 * merge_incore_recursive in merge-ort.h
+		 */
+		merge_bases = get_merge_bases(parent1, parent2);
+		if (!merge_bases && !o->allow_unrelated_histories)
+			die(_("refusing to merge unrelated histories"));
+		merge_bases = reverse_commit_list(merge_bases);
+		merge_incore_recursive(&opt, merge_bases, parent1, parent2, &result);
+	}
 
-	merge_incore_recursive(&opt, merge_bases, parent1, parent2, &result);
 	if (result.clean < 0)
 		die(_("failure to merge"));
 
@@ -478,6 +490,7 @@ static int real_merge(struct merge_tree_options *o,
 
 int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 {
+	const char *merge_base = NULL;
 	struct merge_tree_options o = { .show_messages = -1 };
 	int expected_remaining_argc;
 	int original_argc;
@@ -505,6 +518,10 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 			   &o.allow_unrelated_histories,
 			   N_("allow merging unrelated histories"),
 			   PARSE_OPT_NONEG),
+		OPT_STRING(0, "merge-base",
+			 &merge_base,
+			 N_("commit"),
+			 N_("specify a merge-base for the merge")),
 		OPT_END()
 	};
 
@@ -544,8 +561,14 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 		usage_with_options(merge_tree_usage, mt_options);
 
 	/* Do the relevant type of merge */
-	if (o.mode == MODE_REAL)
+	if (o.mode == MODE_REAL) {
+		if (merge_base) {
+			o.base_commit = lookup_commit_reference_by_name(merge_base);
+			if (!o.base_commit)
+				die(_("could not lookup commit %s"), merge_base);
+		}
 		return real_merge(&o, argv[0], argv[1], prefix);
+	}
 	else
 		return trivial_merge(argv[0], argv[1], argv[2]);
 }
diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
index 013b77144bd..64bfe6f4a41 100755
--- a/t/t4301-merge-tree-write-tree.sh
+++ b/t/t4301-merge-tree-write-tree.sh
@@ -819,4 +819,27 @@ test_expect_success SANITY 'merge-ort fails gracefully in a read-only repository
 	test_must_fail git -C read-only merge-tree side1 side2
 '
 
+test_expect_success 'specify merge-base as parent of branch2' '
+	# Setup
+	git init base-b2-p && (
+		cd base-b2-p &&
+		test_commit c1 file1 &&
+		test_commit c2 file2 &&
+		test_commit c3 file3
+	) &&
+	# Testing
+	(
+		cd base-b2-p &&
+		TREE_OID=$(git merge-tree --write-tree --merge-base=c2 c1 c3) &&
+
+		q_to_tab <<-EOF >expect &&
+		100644 blob $(git rev-parse c1:file1)Qfile1
+		100644 blob $(git rev-parse c3:file3)Qfile3
+		EOF
+
+		git ls-tree $TREE_OID >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done
