diff --git a/Documentation/git-merge-tree.txt b/Documentation/git-merge-tree.txt
index 75b57f8aba..628324646d 100644
--- a/Documentation/git-merge-tree.txt
+++ b/Documentation/git-merge-tree.txt
@@ -59,6 +59,11 @@ OPTIONS
 	default is to include these messages if there are merge
 	conflicts, and to omit them otherwise.
 
+--allow-unrelated-histories::
+	merge-tree will by default error out if the two branches specified
+	share no common history.  This flag can be given to override that
+	check and make the merge proceed anyway.
+
 [[OUTPUT]]
 OUTPUT
 ------
diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
index c159e31774..ae5782917b 100644
--- a/builtin/merge-tree.c
+++ b/builtin/merge-tree.c
@@ -399,6 +399,7 @@ enum mode {
 
 struct merge_tree_options {
 	int mode;
+	int allow_unrelated_histories;
 	int show_messages;
 	int name_only;
 };
@@ -434,7 +435,7 @@ static int real_merge(struct merge_tree_options *o,
 	 * merge_incore_recursive in merge-ort.h
 	 */
 	merge_bases = get_merge_bases(parent1, parent2);
-	if (!merge_bases)
+	if (!merge_bases && !o->allow_unrelated_histories)
 		die(_("refusing to merge unrelated histories"));
 	merge_bases = reverse_commit_list(merge_bases);
 
@@ -500,6 +501,10 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 			   &o.name_only,
 			   N_("list filenames without modes/oids/stages"),
 			   PARSE_OPT_NONEG),
+		OPT_BOOL_F(0, "allow-unrelated-histories",
+			   &o.allow_unrelated_histories,
+			   N_("allow merging unrelated histories"),
+			   PARSE_OPT_NONEG),
 		OPT_END()
 	};
 
diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
index 88e75b18cc..f091259a55 100755
--- a/t/t4301-merge-tree-write-tree.sh
+++ b/t/t4301-merge-tree-write-tree.sh
@@ -44,7 +44,13 @@ test_expect_success setup '
 	git checkout side3 &&
 	git mv numbers sequence &&
 	test_tick &&
-	git commit -m rename-numbers
+	git commit -m rename-numbers &&
+
+	git switch --orphan unrelated &&
+	>something-else &&
+	git add something-else &&
+	test_tick &&
+	git commit -m first-commit
 '
 
 test_expect_success 'Clean merge' '
@@ -215,4 +221,20 @@ test_expect_success 'NUL terminated conflicted file "lines"' '
 	test_cmp expect actual
 '
 
+test_expect_success 'error out by default for unrelated histories' '
+	test_expect_code 128 git merge-tree --write-tree side1 unrelated 2>error &&
+
+	grep "refusing to merge unrelated histories" error
+'
+
+test_expect_success 'can override merge of unrelated histories' '
+	git merge-tree --write-tree --allow-unrelated-histories side1 unrelated >tree &&
+	TREE=$(cat tree) &&
+
+	git rev-parse side1:numbers side1:greeting side1:whatever unrelated:something-else >expect &&
+	git rev-parse $TREE:numbers $TREE:greeting $TREE:whatever $TREE:something-else >actual &&
+
+	test_cmp expect actual
+'
+
 test_done