diff --git a/Makefile b/Makefile
index e3eaa6a543..bc46fd44f5 100644
--- a/Makefile
+++ b/Makefile
@@ -148,6 +148,9 @@ all::
 # is a simplified version of the merge sort used in glibc. This is
 # recommended if Git triggers O(n^2) behavior in your platform's qsort().
 #
+# Define NO_EXTERNAL_GREP if you don't want "git grep" to ever call
+# your external grep (e.g., if your system lacks grep, if its grep is
+# broken, or spawning external process is slower than built-in grep git has).
 
 GIT-VERSION-FILE: .FORCE-GIT-VERSION-FILE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -760,6 +763,9 @@ endif
 ifdef DIR_HAS_BSD_GROUP_SEMANTICS
 	COMPAT_CFLAGS += -DDIR_HAS_BSD_GROUP_SEMANTICS
 endif
+ifdef NO_EXTERNAL_GREP
+	BASIC_CFLAGS += -DNO_EXTERNAL_GREP
+endif
 
 ifeq ($(TCLTK_PATH),)
 NO_TCLTK=NoThanks
diff --git a/builtin-grep.c b/builtin-grep.c
index f4f4ecb11b..ef299108f5 100644
--- a/builtin-grep.c
+++ b/builtin-grep.c
@@ -12,6 +12,14 @@
 #include "builtin.h"
 #include "grep.h"
 
+#ifndef NO_EXTERNAL_GREP
+#ifdef __unix__
+#define NO_EXTERNAL_GREP 0
+#else
+#define NO_EXTERNAL_GREP 1
+#endif
+#endif
+
 /*
  * git grep pathspecs are somewhat different from diff-tree pathspecs;
  * pathname wildcards are allowed.
@@ -153,7 +161,7 @@ static int grep_file(struct grep_opt *opt, const char *filename)
 	return i;
 }
 
-#ifdef __unix__
+#if !NO_EXTERNAL_GREP
 static int exec_grep(int argc, const char **argv)
 {
 	pid_t pid;
@@ -372,7 +380,7 @@ static int grep_cache(struct grep_opt *opt, const char **paths, int cached)
 	int nr;
 	read_cache();
 
-#ifdef __unix__
+#if !NO_EXTERNAL_GREP
 	/*
 	 * Use the external "grep" command for the case where
 	 * we grep through the checked-out files. It tends to