diff --git a/lib/libc/tests/string/Makefile b/lib/libc/tests/string/Makefile
--- a/lib/libc/tests/string/Makefile
+++ b/lib/libc/tests/string/Makefile
@@ -11,6 +11,7 @@
 ATF_TESTS_C+=		flsll_test
 ATF_TESTS_C+=		memcmp_test
 ATF_TESTS_C+=		memset_s_test
+ATF_TESTS_C+=		strncmp_test
 ATF_TESTS_C+=		stpncpy_test
 ATF_TESTS_C+=		strcmp2_test
 ATF_TESTS_C+=		strcspn_test
diff --git a/lib/libc/tests/string/strncmp_test.c b/lib/libc/tests/string/strncmp_test.c
new file mode 100644
--- /dev/null
+++ b/lib/libc/tests/string/strncmp_test.c
@@ -0,0 +1,165 @@
+/*-
+ * Copyright (c) 2023 The FreeBSD Foundation
+ *
+ * This software was developed by Robert Clausecker <fuz@FreeBSD.org>
+ * under sponsorship from the FreeBSD Foundation.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ''AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE
+ */
+
+#include <sys/cdefs.h>
+
+#include <atf-c.h>
+#include <dlfcn.h>
+#include <string.h>
+
+int (*volatile strncmp_fn)(const char *, const char *, size_t);
+
+static void
+alignment_testcase(char *a, char *b, int want, size_t len)
+{
+	int res;
+
+	res = strncmp_fn(a, b, len);
+	ATF_CHECK_MSG(want == (res > 0) - (res < 0),
+	    "strcmp(%p \"%s\", %p \"%s\", %zu) = %d != %d",
+	    (void *)a, a, (void *)b, b, len, res, want);
+}
+
+static void
+check_strncmp_alignments(char a[], char b[],
+    size_t a_off, size_t b_off, size_t len, size_t pos)
+{
+	char *a_str, *b_str, a_orig, b_orig;
+
+	a[a_off] = '\0';
+	b[b_off] = '\0';
+
+	a_str = a + a_off + 1;
+	b_str = b + b_off + 1;
+
+	a_str[len] = '\0';
+	b_str[len] = '\0';
+	a_str[len+1] = 'A';
+	b_str[len+1] = 'B';
+
+	a_orig = a_str[pos];
+	b_orig = b_str[pos];
+
+	alignment_testcase(a_str, b_str, 0, len + 16);
+	alignment_testcase(a_str, b_str, 0, len + 1);
+	alignment_testcase(a_str, b_str, 0, len);
+
+	if (pos < len) {
+		a_str[pos] = '\0';
+		alignment_testcase(a_str, b_str, -1, len + 16);
+		alignment_testcase(a_str, b_str, -1, len + 1);
+		alignment_testcase(a_str, b_str, -1, len);
+		alignment_testcase(a_str, b_str, -1, pos + 1);
+		alignment_testcase(a_str, b_str, 0, pos);
+		a_str[pos] = a_orig;
+
+		b_str[pos] = '\0';
+		alignment_testcase(a_str, b_str, 1, len + 16);
+		alignment_testcase(a_str, b_str, 1, len + 1);
+		alignment_testcase(a_str, b_str, 1, len);
+		alignment_testcase(a_str, b_str, 1, pos + 1);
+		alignment_testcase(a_str, b_str, 0, pos);
+		b_str[pos] = b_orig;
+	}
+
+	a_str[pos] = 'X';
+	alignment_testcase(a_str, b_str, 1, len + 16);
+	alignment_testcase(a_str, b_str, 0, pos);
+	alignment_testcase(a_str, b_str, 1, pos + 1);
+	if (pos < len) {
+		alignment_testcase(a_str, b_str, 1, len);
+		alignment_testcase(a_str, b_str, 1, len + 1);
+	}
+	a_str[pos] = a_orig;
+
+	b_str[pos] = 'X';
+	alignment_testcase(a_str, b_str, -1, len + 16);
+	alignment_testcase(a_str, b_str, 0, pos);
+	alignment_testcase(a_str, b_str, -1, pos + 1);
+	if (pos < len) {
+		alignment_testcase(a_str, b_str, -1, len);
+		alignment_testcase(a_str, b_str, -1, len + 1);
+	}
+	b_str[pos] = b_orig;
+
+	a[a_off] = '-';
+	b[b_off] = '-';
+	a_str[len] = '-';
+	b_str[len] = '-';
+	a_str[len+1] = '-';
+	b_str[len+1] = '-';
+}
+
+ATF_TC(strncmp_alignments);
+ATF_TC_HEAD(strncmp_alignments, tc)
+{
+	atf_tc_set_md_var(tc, "descr", "Test strncmp(3) with various alignments");
+}
+
+ATF_TC_BODY(strncmp_alignments, tc)
+{
+	size_t a_off, b_off, len, pos;
+	char a[64+16+16+3], b[64+16+16+3];
+
+	memset(a, '-', sizeof(a));
+	memset(b, '-', sizeof(b));
+	a[sizeof(a) - 1] = '\0';
+	b[sizeof(b) - 1] = '\0';
+
+	for (a_off = 0; a_off < 16; a_off++)
+		for (b_off = 0; b_off < 16; b_off++)
+			for (len = 1; len <= 64; len++)
+				for (pos = 0; pos <= len; pos++)
+					check_strncmp_alignments(a, b, a_off, b_off, len, pos);
+}
+
+ATF_TC(strncmp_null);
+ATF_TC_HEAD(strncmp_null, tc)
+{
+	atf_tc_set_md_var(tc, "descr", "Test strncmp(3) with null pointers");
+}
+
+ATF_TC_BODY(strncmp_null, tc)
+{
+	alignment_testcase(NULL, NULL, 0, 0);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	void *dl_handle;
+
+	dl_handle = dlopen(NULL, RTLD_LAZY);
+	strncmp_fn = dlsym(dl_handle, "test_strncmp");
+	if (strncmp_fn == NULL)
+		strncmp_fn = strncmp;
+
+	ATF_TP_ADD_TC(tp, strncmp_alignments);
+	ATF_TP_ADD_TC(tp, strncmp_null);
+
+	return atf_no_error();
+}