diff --git a/lib/common/tests/options/Makefile.am b/lib/common/tests/options/Makefile.am
index 204b21a809..bda3b1127e 100644
--- a/lib/common/tests/options/Makefile.am
+++ b/lib/common/tests/options/Makefile.am
@@ -1,23 +1,25 @@
 #
 # 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 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__env_option_test
+check_PROGRAMS = \
+		pcmk__env_option_test \
+		pcmk__set_env_option_test
 
 TESTS = $(check_PROGRAMS)
diff --git a/lib/common/tests/options/pcmk__set_env_option_test.c b/lib/common/tests/options/pcmk__set_env_option_test.c
new file mode 100644
index 0000000000..69ee84bde5
--- /dev/null
+++ b/lib/common/tests/options/pcmk__set_env_option_test.c
@@ -0,0 +1,162 @@
+/*
+ * 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 "mock_private.h"
+
+static void
+bad_input_string(void **state)
+{
+    // Bad setenv()/unsetenv() input: NULL, empty, or containing '='
+
+    // Never call setenv()
+    pcmk__mock_setenv = true;
+
+    pcmk__set_env_option(NULL, "new_value");
+    pcmk__set_env_option("", "new_value");
+    pcmk__set_env_option("name=val", "new_value");
+
+    pcmk__mock_setenv = false;
+
+    // Never call unsetenv()
+    pcmk__mock_unsetenv = true;
+
+    pcmk__set_env_option(NULL, NULL);
+    pcmk__set_env_option("", NULL);
+    pcmk__set_env_option("name=val", NULL);
+
+    pcmk__mock_unsetenv = false;
+}
+
+static void
+input_too_long_for_both(void **state)
+{
+    /* pcmk__set_env_option() wants to set "PCMK_<option>" and "HA_<option>". If
+     * "PCMK_<option>" is too long for the buffer, it simply moves on to
+     * "HA_<option>". A string of length (NAME_MAX - 3) will set us just over
+     * the edge for both tries.
+     */
+    char long_opt[NAME_MAX - 2];
+
+    for (int i = 0; i < NAME_MAX - 3; i++) {
+        long_opt[i] = 'a';
+    }
+    long_opt[NAME_MAX - 3] = '\0';
+
+    // Never call setenv() or unsetenv()
+    pcmk__mock_setenv = true;
+    pcmk__set_env_option(long_opt, "new_value");
+    pcmk__mock_setenv = false;
+
+    pcmk__mock_unsetenv = true;
+    pcmk__set_env_option(long_opt, NULL);
+    pcmk__mock_unsetenv = false;
+}
+
+static void
+input_too_long_for_pcmk(void **state)
+{
+    /* If an input is too long to set "PCMK_<option>", make sure we fall through
+     * to try to set "HA_<option>".
+     *
+     * A string of length (NAME_MAX - 5) will set us just over the edge for
+     * "PCMK_<option>", while still short enough for "HA_<option>" to fit.
+     */
+    char long_opt[NAME_MAX - 4];
+    char buf[NAME_MAX];
+
+    for (int i = 0; i < NAME_MAX - 5; i++) {
+        long_opt[i] = 'a';
+    }
+    long_opt[NAME_MAX - 5] = '\0';
+
+    snprintf(buf, NAME_MAX, "HA_%s", long_opt);
+
+    // Call setenv() for "HA_" only
+    pcmk__mock_setenv = true;
+
+    expect_string(__wrap_setenv, name, buf);
+    expect_string(__wrap_setenv, value, "new_value");
+    expect_value(__wrap_setenv, overwrite, 1);
+    will_return(__wrap_setenv, 0);
+    pcmk__set_env_option(long_opt, "new_value");
+
+    pcmk__mock_setenv = false;
+
+    // Call unsetenv() for "HA_" only
+    pcmk__mock_unsetenv = true;
+
+    expect_string(__wrap_unsetenv, name, buf);
+    will_return(__wrap_unsetenv, 0);
+    pcmk__set_env_option(long_opt, NULL);
+
+    pcmk__mock_unsetenv = false;
+}
+
+static void
+valid_inputs_set(void **state)
+{
+    // Make sure we set "PCMK_<option>" and "HA_<option>"
+    pcmk__mock_setenv = true;
+
+    expect_string(__wrap_setenv, name, "PCMK_env_var");
+    expect_string(__wrap_setenv, value, "new_value");
+    expect_value(__wrap_setenv, overwrite, 1);
+    will_return(__wrap_setenv, 0);
+    expect_string(__wrap_setenv, name, "HA_env_var");
+    expect_string(__wrap_setenv, value, "new_value");
+    expect_value(__wrap_setenv, overwrite, 1);
+    will_return(__wrap_setenv, 0);
+    pcmk__set_env_option("env_var", "new_value");
+
+    // Empty string is also a valid value
+    expect_string(__wrap_setenv, name, "PCMK_env_var");
+    expect_string(__wrap_setenv, value, "");
+    expect_value(__wrap_setenv, overwrite, 1);
+    will_return(__wrap_setenv, 0);
+    expect_string(__wrap_setenv, name, "HA_env_var");
+    expect_string(__wrap_setenv, value, "");
+    expect_value(__wrap_setenv, overwrite, 1);
+    will_return(__wrap_setenv, 0);
+    pcmk__set_env_option("env_var", "");
+
+    pcmk__mock_setenv = false;
+}
+
+static void
+valid_inputs_unset(void **state)
+{
+    // Make sure we unset "PCMK_<option>" and "HA_<option>"
+    pcmk__mock_unsetenv = true;
+
+    expect_string(__wrap_unsetenv, name, "PCMK_env_var");
+    will_return(__wrap_unsetenv, 0);
+    expect_string(__wrap_unsetenv, name, "HA_env_var");
+    will_return(__wrap_unsetenv, 0);
+    pcmk__set_env_option("env_var", NULL);
+
+    pcmk__mock_unsetenv = false;
+}
+
+int
+main(int argc, char **argv)
+{
+    const struct CMUnitTest tests[] = {
+        cmocka_unit_test(bad_input_string),
+        cmocka_unit_test(input_too_long_for_both),
+        cmocka_unit_test(input_too_long_for_pcmk),
+        cmocka_unit_test(valid_inputs_set),
+        cmocka_unit_test(valid_inputs_unset),
+    };
+
+    cmocka_set_message_output(CM_OUTPUT_TAP);
+    return cmocka_run_group_tests(tests, NULL, NULL);
+}