diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 0f07c9c4a8..3d2b9d0a06 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -168,5 +168,13 @@
 --quiet::
 	Disable all output of the program. Implies --exit-code.
 
+--ext-diff::
+	Allow an external diff helper to be executed. If you set an
+	external diff driver with gitlink:gitattributes(5), you need
+	to use this option with gitlink:git-log(1) and friends.
+
+--no-ext-diff::
+	Disallow external diff drivers.
+
 For more detailed explanation on these common options, see also
 link:diffcore.html[diffcore documentation].
diff --git a/diff.c b/diff.c
index 93eca79c17..b6eb72be02 100644
--- a/diff.c
+++ b/diff.c
@@ -2241,6 +2241,10 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		options->exit_with_status = 1;
 	else if (!strcmp(arg, "--quiet"))
 		options->quiet = 1;
+	else if (!strcmp(arg, "--ext-diff"))
+		options->allow_external = 1;
+	else if (!strcmp(arg, "--no-ext-diff"))
+		options->allow_external = 0;
 	else
 		return 0;
 	return 1;