diff --git a/cache.h b/cache.h
index 98af53040d..e1f94cbade 100644
--- a/cache.h
+++ b/cache.h
@@ -214,6 +214,7 @@ extern char *get_object_directory(void);
 extern char *get_refs_directory(void);
 extern char *get_index_file(void);
 extern char *get_graft_file(void);
+extern int set_git_dir(const char *path);
 
 #define ALTERNATE_DB_ENVIRONMENT "GIT_ALTERNATE_OBJECT_DIRECTORIES"
 
diff --git a/environment.c b/environment.c
index f83fb9e448..a571fae607 100644
--- a/environment.c
+++ b/environment.c
@@ -107,3 +107,11 @@ char *get_graft_file(void)
 		setup_git_env();
 	return git_graft_file;
 }
+
+int set_git_dir(const char *path)
+{
+	if (setenv(GIT_DIR_ENVIRONMENT, path, 1))
+		return error("Could not set GIT_DIR to '%s'", path);
+	setup_git_env();
+	return 0;
+}