diff --git a/etc/mtree/BSD.tests.dist b/etc/mtree/BSD.tests.dist
--- a/etc/mtree/BSD.tests.dist
+++ b/etc/mtree/BSD.tests.dist
@@ -1083,6 +1083,8 @@
         ..
         locale
         ..
+        lockf
+        ..
         m4
         ..
         mkimg
diff --git a/usr.bin/lockf/Makefile b/usr.bin/lockf/Makefile
--- a/usr.bin/lockf/Makefile
+++ b/usr.bin/lockf/Makefile
@@ -1,4 +1,9 @@
 
+.include <src.opts.mk>
+
 PROG=	lockf
 
+HAS_TESTS=
+SUBDIR.${MK_TESTS}+= tests
+
 .include <bsd.prog.mk>
diff --git a/usr.bin/lockf/tests/Makefile b/usr.bin/lockf/tests/Makefile
new file mode 100644
--- /dev/null
+++ b/usr.bin/lockf/tests/Makefile
@@ -0,0 +1,6 @@
+
+PACKAGE=	tests
+
+ATF_TESTS_SH+=	lockf_test
+
+.include <bsd.test.mk>
diff --git a/usr.bin/lockf/tests/lockf_test.sh b/usr.bin/lockf/tests/lockf_test.sh
new file mode 100644
--- /dev/null
+++ b/usr.bin/lockf/tests/lockf_test.sh
@@ -0,0 +1,148 @@
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+# Copyright (c) 2023 Klara, Inc.
+#
+# 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.
+#
+
+# sysexits(3)
+: ${EX_USAGE:=64}
+: ${EX_UNAVAILABLE:=69}
+: ${EX_CANTCREAT:=73}
+: ${EX_TEMPFAIL:=75}
+
+atf_test_case badargs
+badargs_body()
+{
+	atf_check -s exit:${EX_USAGE} -e not-empty lockf
+	atf_check -s exit:${EX_USAGE} -e not-empty lockf "testlock"
+}
+
+atf_test_case basic
+basic_body()
+{
+	# Something innocent so that it does eventually go away without our
+	# intervention.
+	lockf "testlock" sleep 10 &
+	lpid=$!
+
+	# Make sure that the lock exists...
+	atf_check test -e "testlock"
+
+	# Attempt both verbose and silent re-lock
+	atf_check -s exit:${EX_TEMPFAIL} -e not-empty \
+	    lockf -t 0 "testlock" sleep 0
+	atf_check -s exit:${EX_TEMPFAIL} -e empty \
+	    lockf -t 0 -s "testlock" sleep 0
+
+	# Make sure it cleans up after the initial sleep 10 is over.
+	wait "$lpid"
+	atf_check test ! -e "testlock"
+}
+
+atf_test_case keep
+keep_body()
+{
+	lockf -k "testlock" sleep 10 &
+	lpid=$!
+
+	# Make sure that the lock exists now...
+	while ! test -e "testlock"; do
+		sleep 0.5
+	done
+
+	kill "$lpid"
+	wait "$lpid"
+
+	# And it still exits after the lock has been relinquished.
+	atf_check test -e "testlock"
+}
+
+atf_test_case needfile
+needfile_body()
+{
+	# Hopefully the clock doesn't jump.
+	start=$(date +"%s")
+
+	# Should fail if the lockfile does not yet exist.
+	atf_check -s exit:"${EX_UNAVAILABLE}" lockf -sn "testlock" sleep 30
+
+	# It's hard to guess how quickly we should have finished that; one would
+	# hope that it exits fast, but to be safe we specified a sleep 30 under
+	# lock so that we have a good margin below that duration that we can
+	# safely test to make sure we didn't actually execute the program, more
+	# or less.
+	now=$(date +"%s")
+	tpass=$((now - start))
+	atf_check test "$tpass" -lt 10
+}
+
+atf_test_case timeout
+timeout_body()
+{
+	lockf "testlock" sleep 30 &
+	lpid=$!
+
+	while ! test -e "testlock"; do
+		sleep 0.5
+	done
+
+	start=$(date +"%s")
+	timeout=2
+	atf_check -s exit:${EX_TEMPFAIL} lockf -st "$timeout" "testlock" sleep 0
+
+	# We should have taken no less than our timeout, at least.
+	now=$(date +"%s")
+	tpass=$((now - start))
+	atf_check test "$tpass" -ge "$timeout"
+
+	kill "$lpid"
+	wait "$lpid" || true
+}
+
+atf_test_case wrlock
+wrlock_head()
+{
+	atf_set "require.user" "unprivileged"
+}
+wrlock_body()
+{
+	touch "testlock"
+	chmod -w "testlock"
+
+	# Demonstrate that we can lock the file normally, but -w fails if we
+	# can't write.
+	atf_check lockf -kt 0 "testlock" sleep 0
+	atf_check -s exit:${EX_CANTCREAT} -e not-empty \
+	    lockf -wt 0 "testlock" sleep 0
+}
+
+atf_init_test_cases()
+{
+	atf_add_test_case badargs
+	atf_add_test_case basic
+	atf_add_test_case keep
+	atf_add_test_case needfile
+	atf_add_test_case timeout
+	atf_add_test_case wrlock
+}