diff --git a/Makefile b/Makefile
index d3a3f16f076..5418ddd03be 100644
--- a/Makefile
+++ b/Makefile
@@ -1347,6 +1347,7 @@ UNIT_TEST_PROGRAMS += t-ctype
 UNIT_TEST_PROGRAMS += t-mem-pool
 UNIT_TEST_PROGRAMS += t-prio-queue
 UNIT_TEST_PROGRAMS += t-strbuf
+UNIT_TEST_PROGRAMS += t-trailer
 UNIT_TEST_PROGS = $(patsubst %,$(UNIT_TEST_BIN)/%$X,$(UNIT_TEST_PROGRAMS))
 UNIT_TEST_OBJS = $(patsubst %,$(UNIT_TEST_DIR)/%.o,$(UNIT_TEST_PROGRAMS))
 UNIT_TEST_OBJS += $(UNIT_TEST_DIR)/test-lib.o
diff --git a/t/unit-tests/t-trailer.c b/t/unit-tests/t-trailer.c
new file mode 100644
index 00000000000..c1f897235c7
--- /dev/null
+++ b/t/unit-tests/t-trailer.c
@@ -0,0 +1,174 @@
+#include "test-lib.h"
+#include "trailer.h"
+
+static void t_trailer_iterator(const char *msg, size_t num_expected_trailers)
+{
+	struct trailer_iterator iter;
+	size_t i = 0;
+
+	trailer_iterator_init(&iter, msg);
+	while (trailer_iterator_advance(&iter))
+		i++;
+	trailer_iterator_release(&iter);
+
+	check_uint(i, ==, num_expected_trailers);
+}
+
+static void run_t_trailer_iterator(void)
+{
+	static struct test_cases {
+		const char *name;
+		const char *msg;
+		size_t num_expected_trailers;
+	} tc[] = {
+		{
+			"empty input",
+			"",
+			0
+		},
+		{
+			"no newline at beginning",
+			"Fixes: x\n"
+			"Acked-by: x\n"
+			"Reviewed-by: x\n",
+			0
+		},
+		{
+			"newline at beginning",
+			"\n"
+			"Fixes: x\n"
+			"Acked-by: x\n"
+			"Reviewed-by: x\n",
+			3
+		},
+		{
+			"without body text",
+			"subject: foo bar\n"
+			"\n"
+			"Fixes: x\n"
+			"Acked-by: x\n"
+			"Reviewed-by: x\n",
+			3
+		},
+		{
+			"with body text, without divider",
+			"my subject\n"
+			"\n"
+			"my body which is long\n"
+			"and contains some special\n"
+			"chars like : = ? !\n"
+			"hello\n"
+			"\n"
+			"Fixes: x\n"
+			"Acked-by: x\n"
+			"Reviewed-by: x\n"
+			"Signed-off-by: x\n",
+			4
+		},
+		{
+			"with body text, without divider (second trailer block)",
+			"my subject\n"
+			"\n"
+			"my body which is long\n"
+			"and contains some special\n"
+			"chars like : = ? !\n"
+			"hello\n"
+			"\n"
+			"Fixes: x\n"
+			"Acked-by: x\n"
+			"Reviewed-by: x\n"
+			"Signed-off-by: x\n"
+			"\n"
+			/*
+			 * Because this is the last trailer block, it takes
+			 * precedence over the first one encountered above.
+			 */
+			"Helped-by: x\n"
+			"Signed-off-by: x\n",
+			2
+		},
+		{
+			"with body text, with divider",
+			"my subject\n"
+			"\n"
+			"my body which is long\n"
+			"and contains some special\n"
+			"chars like : = ? !\n"
+			"hello\n"
+			"\n"
+			"---\n"
+			"\n"
+			/*
+			 * This trailer still counts because the iterator
+			 * always ignores the divider.
+			 */
+			"Signed-off-by: x\n",
+			1
+		},
+		{
+			"with non-trailer lines in trailer block",
+			"subject: foo bar\n"
+			"\n"
+			/*
+			 * Even though this trailer block has a non-trailer line
+			 * in it, it's still a valid trailer block because it's
+			 * at least 25% trailers and is Git-generated (see
+			 * git_generated_prefixes[] in trailer.c).
+			 */
+			"not a trailer line\n"
+			"not a trailer line\n"
+			"not a trailer line\n"
+			"Signed-off-by: x\n",
+			1
+		},
+		{
+			"with non-trailer lines (one too many) in trailer block",
+			"subject: foo bar\n"
+			"\n"
+			/*
+			 * This block has only 20% trailers, so it's below the
+			 * 25% threshold.
+			 */
+			"not a trailer line\n"
+			"not a trailer line\n"
+			"not a trailer line\n"
+			"not a trailer line\n"
+			"Signed-off-by: x\n",
+			0
+		},
+		{
+			"with non-trailer lines (only 1) in trailer block, but no Git-generated trailers",
+			"subject: foo bar\n"
+			"\n"
+			/*
+			 * This block has only 1 non-trailer out of 10 (IOW, 90%
+			 * trailers) but is not considered a trailer block
+			 * because the 25% threshold only applies to cases where
+			 * there was a Git-generated trailer.
+			 */
+			"Reviewed-by: x\n"
+			"Reviewed-by: x\n"
+			"Reviewed-by: x\n"
+			"Helped-by: x\n"
+			"Helped-by: x\n"
+			"Helped-by: x\n"
+			"Acked-by: x\n"
+			"Acked-by: x\n"
+			"Acked-by: x\n"
+			"not a trailer line\n",
+			0
+		},
+	};
+
+	for (int i = 0; i < sizeof(tc) / sizeof(tc[0]); i++) {
+		TEST(t_trailer_iterator(tc[i].msg,
+					tc[i].num_expected_trailers),
+		     "%s", tc[i].name);
+	}
+}
+
+int cmd_main(int argc, const char **argv)
+{
+	run_t_trailer_iterator();
+	return test_done();
+}
