diff --git a/merge-recursive.c b/merge-recursive.c
index 3b34401d0b25126b7955c0d44dad3e69a9592129..8ec18ad5779c5c1de254411167533eb6ac55089f 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -15,6 +15,7 @@
 #include "unpack-trees.h"
 #include "path-list.h"
 #include "xdiff-interface.h"
+#include "interpolate.h"
 #include "attr.h"
 
 static int subtree_merge;
@@ -661,12 +662,14 @@ static void fill_mm(const unsigned char *sha1, mmfile_t *mm)
 }
 
 /* Low-level merge functions */
-typedef int (*ll_merge_fn)(mmfile_t *orig,
+typedef int (*ll_merge_fn)(const char *cmd,
+			   mmfile_t *orig,
 			   mmfile_t *src1, const char *name1,
 			   mmfile_t *src2, const char *name2,
 			   mmbuffer_t *result);
 
-static int ll_xdl_merge(mmfile_t *orig,
+static int ll_xdl_merge(const char *cmd__unused,
+			mmfile_t *orig,
 			mmfile_t *src1, const char *name1,
 			mmfile_t *src2, const char *name2,
 			mmbuffer_t *result)
@@ -681,7 +684,8 @@ static int ll_xdl_merge(mmfile_t *orig,
 			 result);
 }
 
-static int ll_union_merge(mmfile_t *orig,
+static int ll_union_merge(const char *cmd__unused,
+			  mmfile_t *orig,
 			  mmfile_t *src1, const char *name1,
 			  mmfile_t *src2, const char *name2,
 			  mmbuffer_t *result)
@@ -690,7 +694,8 @@ static int ll_union_merge(mmfile_t *orig,
 	long size;
 	const int marker_size = 7;
 
-	int status = ll_xdl_merge(orig, src1, NULL, src2, NULL, result);
+	int status = ll_xdl_merge(cmd__unused, orig,
+				  src1, NULL, src2, NULL, result);
 	if (status <= 0)
 		return status;
 	size = result->size;
@@ -721,7 +726,8 @@ static int ll_union_merge(mmfile_t *orig,
 	return 0;
 }
 
-static int ll_binary_merge(mmfile_t *orig,
+static int ll_binary_merge(const char *cmd__unused,
+			   mmfile_t *orig,
 			   mmfile_t *src1, const char *name1,
 			   mmfile_t *src2, const char *name2,
 			   mmbuffer_t *result)
@@ -743,24 +749,169 @@ static struct {
 	const char *name;
 	ll_merge_fn fn;
 } ll_merge_fns[] = {
-	{ "text", ll_xdl_merge },
 	{ "binary", ll_binary_merge },
+	{ "text", ll_xdl_merge },
 	{ "union", ll_union_merge },
 	{ NULL, NULL },
 };
 
-static ll_merge_fn find_ll_merge_fn(void *merge_attr)
+static void create_temp(mmfile_t *src, char *path)
 {
+	int fd;
+
+	strcpy(path, ".merge_file_XXXXXX");
+	fd = mkstemp(path);
+	if (fd < 0)
+		die("unable to create temp-file");
+	if (write_in_full(fd, src->ptr, src->size) != src->size)
+		die("unable to write temp-file");
+	close(fd);
+}
+
+static int ll_ext_merge(const char *cmd,
+			mmfile_t *orig,
+			mmfile_t *src1, const char *name1,
+			mmfile_t *src2, const char *name2,
+			mmbuffer_t *result)
+{
+	char temp[3][50];
+	char cmdbuf[2048];
+	struct interp table[] = {
+		{ "%O" },
+		{ "%A" },
+		{ "%B" },
+	};
+	struct child_process child;
+	const char *args[20];
+	int status, fd, i;
+	struct stat st;
+
+	result->ptr = NULL;
+	result->size = 0;
+	create_temp(orig, temp[0]);
+	create_temp(src1, temp[1]);
+	create_temp(src2, temp[2]);
+
+	interp_set_entry(table, 0, temp[0]);
+	interp_set_entry(table, 1, temp[1]);
+	interp_set_entry(table, 2, temp[2]);
+
+	interpolate(cmdbuf, sizeof(cmdbuf), cmd, table, 3);
+
+	memset(&child, 0, sizeof(child));
+	child.argv = args;
+	args[0] = "sh";
+	args[1] = "-c";
+	args[2] = cmdbuf;
+	args[3] = NULL;
+
+	status = run_command(&child);
+	if (status < -ERR_RUN_COMMAND_FORK)
+		; /* failure in run-command */
+	else
+		status = -status;
+	fd = open(temp[1], O_RDONLY);
+	if (fd < 0)
+		goto bad;
+	if (fstat(fd, &st))
+		goto close_bad;
+	result->size = st.st_size;
+	result->ptr = xmalloc(result->size + 1);
+	if (read_in_full(fd, result->ptr, result->size) != result->size) {
+		free(result->ptr);
+		result->ptr = NULL;
+		result->size = 0;
+	}
+ close_bad:
+	close(fd);
+ bad:
+	for (i = 0; i < 3; i++)
+		unlink(temp[i]);
+	return status;
+}
+
+/*
+ * merge.default and merge.driver configuration items
+ */
+static struct user_merge_fn {
+	struct user_merge_fn *next;
+	const char *name;
+	char *cmdline;
+	char b_[1];
+} *ll_user_merge_fns, **ll_user_merge_fns_tail;
+
+static int read_merge_config(const char *var, const char *value)
+{
+	struct user_merge_fn *fn;
+	int blen, nlen;
+
+	if (strcmp(var, "merge.driver"))
+		return 0;
+	if (!value)
+		return error("%s: lacks value", var);
+	/*
+	 * merge.driver is a multi-valued configuration, whose value is
+	 * of form:
+	 *
+	 *	name command-line
+	 *
+	 * The command-line will be interpolated with the following
+	 * tokens and is given to the shell:
+	 *
+	 *    %O - temporary file name for the merge base.
+	 *    %A - temporary file name for our version.
+	 *    %B - temporary file name for the other branches' version.
+	 *
+	 * The external merge driver should write the results in the file
+	 * named by %A, and signal that it has done with exit status 0.
+	 */
+	for (nlen = -1, blen = 0; value[blen]; blen++)
+		if (nlen < 0 && isspace(value[blen]))
+			nlen = blen;
+	if (nlen < 0)
+		return error("%s '%s': lacks command line", var, value);
+	fn = xcalloc(1, sizeof(struct user_merge_fn) + blen + 1);
+	memcpy(fn->b_, value, blen + 1);
+	fn->name = fn->b_;
+	fn->b_[nlen] = 0;
+	fn->cmdline = fn->b_ + nlen + 1;
+	fn->next = *ll_user_merge_fns_tail;
+	*ll_user_merge_fns_tail = fn;
+	return 0;
+}
+
+static void initialize_ll_merge(void)
+{
+	if (ll_user_merge_fns_tail)
+		return;
+	ll_user_merge_fns_tail = &ll_user_merge_fns;
+	git_config(read_merge_config);
+}
+
+static ll_merge_fn find_ll_merge_fn(void *merge_attr, const char **cmdline)
+{
+	struct user_merge_fn *fn;
 	const char *name;
 	int i;
 
-	if (ATTR_TRUE(merge_attr) || ATTR_UNSET(merge_attr))
+	initialize_ll_merge();
+
+	if (ATTR_TRUE(merge_attr))
 		return ll_xdl_merge;
 	else if (ATTR_FALSE(merge_attr))
 		return ll_binary_merge;
+	else if (ATTR_UNSET(merge_attr))
+		return ll_xdl_merge;
+	else
+		name = merge_attr;
+
+	for (fn = ll_user_merge_fns; fn; fn = fn->next) {
+		if (!strcmp(fn->name, name)) {
+			*cmdline = fn->cmdline;
+			return ll_ext_merge;
+		}
+	}
 
-	/* Otherwise merge_attr may name the merge function */
-	name = merge_attr;
 	for (i = 0; ll_merge_fns[i].name; i++)
 		if (!strcmp(ll_merge_fns[i].name, name))
 			return ll_merge_fns[i].fn;
@@ -793,6 +944,7 @@ static int ll_merge(mmbuffer_t *result_buf,
 	int merge_status;
 	void *merge_attr;
 	ll_merge_fn fn;
+	const char *driver = NULL;
 
 	name1 = xstrdup(mkpath("%s:%s", branch1, a->path));
 	name2 = xstrdup(mkpath("%s:%s", branch2, b->path));
@@ -802,9 +954,10 @@ static int ll_merge(mmbuffer_t *result_buf,
 	fill_mm(b->sha1, &src2);
 
 	merge_attr = git_path_check_merge(a->path);
-	fn = find_ll_merge_fn(merge_attr);
+	fn = find_ll_merge_fn(merge_attr, &driver);
 
-	merge_status = fn(&orig, &src1, name1, &src2, name2, result_buf);
+	merge_status = fn(driver, &orig,
+			  &src1, name1, &src2, name2, result_buf);
 
 	free(name1);
 	free(name2);
diff --git a/t/t6026-merge-attr.sh b/t/t6026-merge-attr.sh
index 5daa2236de227c38a2d0d873cd94862fbfe2d549..1732b60ed8be3c8bd8ea39afc62205db4fccc228 100755
--- a/t/t6026-merge-attr.sh
+++ b/t/t6026-merge-attr.sh
@@ -30,8 +30,9 @@ test_expect_success setup '
 		echo Side >>$f && git add $f || break
 	done &&
 	test_tick &&
-	git commit -m Side
+	git commit -m Side &&
 
+	git tag anchor
 '
 
 test_expect_success merge '
@@ -69,4 +70,72 @@ test_expect_success 'check merge result in working tree' '
 
 '
 
+cat >./custom-merge <<\EOF
+#!/bin/sh
+
+orig="$1" ours="$2" theirs="$3" exit="$4"
+(
+	echo "orig is $orig"
+	echo "ours is $ours"
+	echo "theirs is $theirs"
+	echo "=== orig ==="
+	cat "$orig"
+	echo "=== ours ==="
+	cat "$ours"
+	echo "=== theirs ==="
+	cat "$theirs"
+) >"$ours+"
+cat "$ours+" >"$ours"
+rm -f "$ours+"
+exit "$exit"
+EOF
+chmod +x ./custom-merge
+
+test_expect_success 'custom merge backend' '
+
+	echo "* merge=union" >.gitattributes &&
+	echo "text merge=custom" >>.gitattributes &&
+
+	git reset --hard anchor &&
+	git config --replace-all \
+	merge.driver "custom ./custom-merge %O %A %B 0" &&
+
+	git merge master &&
+
+	cmp binary union &&
+	sed -e 1,3d text >check-1 &&
+	o=$(git-unpack-file master^:text) &&
+	a=$(git-unpack-file side^:text) &&
+	b=$(git-unpack-file master:text) &&
+	sh -c "./custom-merge $o $a $b 0" &&
+	sed -e 1,3d $a >check-2 &&
+	cmp check-1 check-2 &&
+	rm -f $o $a $b
+'
+
+test_expect_success 'custom merge backend' '
+
+	git reset --hard anchor &&
+	git config --replace-all \
+	merge.driver "custom ./custom-merge %O %A %B 1" &&
+
+	if git merge master
+	then
+		echo "Eh? should have conflicted"
+		false
+	else
+		echo "Ok, conflicted"
+	fi &&
+
+	cmp binary union &&
+	sed -e 1,3d text >check-1 &&
+	o=$(git-unpack-file master^:text) &&
+	a=$(git-unpack-file anchor:text) &&
+	b=$(git-unpack-file master:text) &&
+	sh -c "./custom-merge $o $a $b 0" &&
+	sed -e 1,3d $a >check-2 &&
+	cmp check-1 check-2 &&
+	rm -f $o $a $b
+'
+
 test_done