diff --git a/lib/common/tests/output/Makefile.am b/lib/common/tests/output/Makefile.am
index 2ee0b0d9ea..490e32b338 100644
--- a/lib/common/tests/output/Makefile.am
+++ b/lib/common/tests/output/Makefile.am
@@ -1,26 +1,27 @@
 #
 # Copyright 2021-2022 the Pacemaker project contributors
 #
 # The version control history for this file may have further details.
 #
 # This source code is licensed under the GNU General Public License version 2
 # or later (GPLv2+) WITHOUT ANY WARRANTY.
 #
 
 AM_CPPFLAGS = -I$(top_srcdir)/include		\
 	      -I$(top_builddir)/include		\
 	      -I$(top_srcdir)/lib/common
 LDADD = $(top_builddir)/lib/common/libcrmcommon_test.la \
 		-lcmocka
 AM_CFLAGS = -DPCMK__UNIT_TESTING
 AM_LDFLAGS = $(LDFLAGS_WRAP)
 
 include $(top_srcdir)/mk/tap.mk
 
 # Add "_test" to the end of all test program names to simplify .gitignore.
 check_PROGRAMS =	pcmk__output_new_test \
 					pcmk__register_format_test \
 					pcmk__register_formats_test \
+					pcmk__register_message_test \
 					pcmk__unregister_formats_test
 
 TESTS = $(check_PROGRAMS)
diff --git a/lib/common/tests/output/pcmk__register_message_test.c b/lib/common/tests/output/pcmk__register_message_test.c
new file mode 100644
index 0000000000..f65a51d911
--- /dev/null
+++ b/lib/common/tests/output/pcmk__register_message_test.c
@@ -0,0 +1,113 @@
+/*
+ * Copyright 2022 the Pacemaker project contributors
+ *
+ * The version control history for this file may have further details.
+ *
+ * This source code is licensed under the GNU Lesser General Public License
+ * version 2.1 or later (LGPLv2.1+) WITHOUT ANY WARRANTY.
+ */
+
+#include <crm_internal.h>
+
+#include <crm/common/unittest_internal.h>
+#include <crm/common/output_internal.h>
+
+static int
+null_message_fn(pcmk__output_t *out, va_list args) {
+    return pcmk_rc_ok;
+}
+
+static int
+null_message_fn_2(pcmk__output_t *out, va_list args) {
+    return pcmk_rc_ok;
+}
+
+static bool
+fake_text_init(pcmk__output_t *out) {
+    return true;
+}
+
+static void
+fake_text_free_priv(pcmk__output_t *out) {
+    /* This function intentionally left blank */
+}
+
+static pcmk__output_t *
+mk_fake_text_output(char **argv) {
+    pcmk__output_t *retval = calloc(1, sizeof(pcmk__output_t));
+
+    if (retval == NULL) {
+        return NULL;
+    }
+
+    retval->fmt_name = "text";
+    retval->init = fake_text_init;
+    retval->free_priv = fake_text_free_priv;
+
+    retval->register_message = pcmk__register_message;
+    retval->message = pcmk__call_message;
+
+    return retval;
+}
+
+static int
+setup(void **state) {
+    pcmk__register_format(NULL, "text", mk_fake_text_output, NULL);
+    return 0;
+}
+
+static int
+teardown(void **state) {
+    pcmk__unregister_formats();
+    return 0;
+}
+
+static void
+null_params(void **state) {
+    pcmk__output_t *out = NULL;
+
+    pcmk__output_new(&out, "text", NULL, NULL);
+
+    pcmk__assert_asserts(pcmk__register_message(NULL, "fake", null_message_fn));
+    pcmk__assert_asserts(pcmk__register_message(out, NULL, null_message_fn));
+    pcmk__assert_asserts(pcmk__register_message(out, "", null_message_fn));
+    pcmk__assert_asserts(pcmk__register_message(out, "fake", NULL));
+
+    pcmk__output_free(out);
+}
+
+static void
+add_message(void **state) {
+    pcmk__output_t *out = NULL;
+
+    pcmk__output_new(&out, "text", NULL, NULL);
+
+    /* For starters, there should be no messages defined. */
+    assert_int_equal(g_hash_table_size(out->messages), 0);
+
+    /* Add a fake function and check that it's the only item in the hash table. */
+    pcmk__register_message(out, "fake", null_message_fn);
+    assert_int_equal(g_hash_table_size(out->messages), 1);
+    assert_ptr_equal(g_hash_table_lookup(out->messages, "fake"), null_message_fn);
+
+    /* Add a second fake function which should overwrite the first one, leaving
+     * only one item in the hash table but pointing at the new function.
+     */
+    pcmk__register_message(out, "fake", null_message_fn_2);
+    assert_int_equal(g_hash_table_size(out->messages), 1);
+    assert_ptr_equal(g_hash_table_lookup(out->messages, "fake"), null_message_fn_2);
+
+    pcmk__output_free(out);
+}
+
+int
+main(int argc, char **argv)
+{
+    const struct CMUnitTest tests[] = {
+        cmocka_unit_test_setup_teardown(null_params, setup, teardown),
+        cmocka_unit_test_setup_teardown(add_message, setup, teardown),
+    };
+
+    cmocka_set_message_output(CM_OUTPUT_TAP);
+    return cmocka_run_group_tests(tests, NULL, NULL);
+}