diff --git a/src/test/test_util.c b/src/test/test_util.c
index 5fe9617f89..1bbe747ec1 100644
--- a/src/test/test_util.c
+++ b/src/test/test_util.c
@@ -357,6 +357,55 @@ test_util_write_chunks_to_file(void *arg)
   tor_free(temp_str);
 }
 
+/* Test write_str_to_file_if_not_equal(). */
+static void
+test_util_write_str_if_changed(void *arg)
+{
+  (void)arg;
+  char *fname = tor_strdup(get_fname("write_if_changed"));
+  char *s = NULL;
+  int rv;
+  const char str1[] = "The wombat lives across the seas";
+  const char str2[] = "Among the far Antipodes"; /* -- Ogden Nash */
+
+  /* We can create files. */
+  rv = write_str_to_file_if_not_equal(fname, str1);
+  tt_int_op(rv, OP_EQ, 0);
+  s = read_file_to_str(fname, 0, NULL);
+  tt_str_op(s, OP_EQ, str1);
+  tor_free(s);
+
+  /* We can replace files. */
+  rv = write_str_to_file_if_not_equal(fname, str2);
+  tt_int_op(rv, OP_EQ, 0);
+  s = read_file_to_str(fname, 0, NULL);
+  tt_str_op(s, OP_EQ, str2);
+  tor_free(s);
+
+  /* Make sure we don't replace files when they're equal. (That's the whole
+   * point of the function we're testing. */
+  /* First, change the mtime of the file so that we can tell whether we
+   * replaced it. */
+  const time_t now = time(NULL);
+  const time_t five_sec_ago = now - 5;
+  set_file_mtime(fname, five_sec_ago);
+  rv = write_str_to_file_if_not_equal(fname, str2);
+  tt_int_op(rv, OP_EQ, 0);
+  /* Make sure that the file's mtime is unchanged... */
+  struct stat st;
+  rv = stat(fname, &st);
+  tt_int_op(rv, OP_EQ, 0);
+  tt_i64_op(st.st_mtime, OP_EQ, five_sec_ago);
+  /* And make sure its contents are unchanged. */
+  s = read_file_to_str(fname, 0, NULL);
+  tt_str_op(s, OP_EQ, str2);
+  tor_free(s);
+
+ done:
+  tor_free(fname);
+  tor_free(s);
+}
+
 #ifndef COCCI
 #define _TFE(a, b, f)  tt_int_op((a).f, OP_EQ, (b).f)
 /** test the minimum set of struct tm fields needed for a unique epoch value
@@ -6557,6 +6606,7 @@ struct testcase_t util_tests[] = {
   UTIL_TEST(read_file_eof_zero_bytes, 0),
   UTIL_TEST(read_file_endlines, 0),
   UTIL_TEST(write_chunks_to_file, 0),
+  UTIL_TEST(write_str_if_changed, 0),
   UTIL_TEST(mathlog, 0),
   UTIL_TEST(fraction, 0),
   UTIL_TEST(weak_random, 0),