diff --git a/integration-cli/docker_cli_run_test.go b/integration-cli/docker_cli_run_test.go
index 43052d13b6..5af5ce28ca 100644
--- a/integration-cli/docker_cli_run_test.go
+++ b/integration-cli/docker_cli_run_test.go
@@ -4290,3 +4290,21 @@ func (s *DockerSuite) TestRunVolumeCopyFlag(c *check.C) {
 	out, _, err = dockerCmdWithError("run", "-v", "/foo:/bar:nocopy", "busybox", "true")
 	c.Assert(err, checker.NotNil, check.Commentf(out))
 }
+
+func (s *DockerSuite) TestRunTooLongHostname(c *check.C) {
+	// Test case in #21445
+	hostname1 := "this-is-a-way-too-long-hostname-but-it-should-give-a-nice-error.local"
+	out, _, err := dockerCmdWithError("run", "--hostname", hostname1, "busybox", "echo", "test")
+	c.Assert(err, checker.NotNil, check.Commentf("Expected docker run to fail!"))
+	c.Assert(out, checker.Contains, "invalid hostname format for --hostname:", check.Commentf("Expected to have 'invalid hostname format for --hostname:' in the output, get: %s!", out))
+
+	// HOST_NAME_MAX=64 so 65 bytes will fail
+	hostname2 := "this-is-a-hostname-with-65-bytes-so-it-should-give-an-error.local"
+	out, _, err = dockerCmdWithError("run", "--hostname", hostname2, "busybox", "echo", "test")
+	c.Assert(err, checker.NotNil, check.Commentf("Expected docker run to fail!"))
+	c.Assert(out, checker.Contains, "invalid hostname format for --hostname:", check.Commentf("Expected to have 'invalid hostname format for --hostname:' in the output, get: %s!", out))
+
+	// 64 bytes will be OK
+	hostname3 := "this-is-a-hostname-with-64-bytes-so-will-not-give-an-error.local"
+	dockerCmd(c, "run", "--hostname", hostname3, "busybox", "echo", "test")
+}
diff --git a/runconfig/opts/parse.go b/runconfig/opts/parse.go
index 722b8d8b3a..6543b406df 100644
--- a/runconfig/opts/parse.go
+++ b/runconfig/opts/parse.go
@@ -244,8 +244,9 @@ func Parse(cmd *flag.FlagSet, args []string) (*container.Config, *container.Host
 	// Validate if the given hostname is RFC 1123 (https://tools.ietf.org/html/rfc1123) compliant.
 	hostname := *flHostname
 	if hostname != "" {
+		// Linux hostname is limited to HOST_NAME_MAX=64, not not including the terminating null byte.
 		matched, _ := regexp.MatchString("^(([[:alnum:]]|[[:alnum:]][[:alnum:]\\-]*[[:alnum:]])\\.)*([[:alnum:]]|[[:alnum:]][[:alnum:]\\-]*[[:alnum:]])$", hostname)
-		if !matched {
+		if len(hostname) > 64 || !matched {
 			return nil, nil, nil, cmd, fmt.Errorf("invalid hostname format for --hostname: %s", hostname)
 		}
 	}
diff --git a/runconfig/opts/parse_test.go b/runconfig/opts/parse_test.go
index b88944f32a..1885544317 100644
--- a/runconfig/opts/parse_test.go
+++ b/runconfig/opts/parse_test.go
@@ -390,6 +390,7 @@ func TestParseHostname(t *testing.T) {
 		"host-name":   "host-name",
 		"hostname123": "hostname123",
 		"123hostname": "123hostname",
+		"hostname-of-64-bytes-long-should-be-valid-and-without-any-errors": "hostname-of-64-bytes-long-should-be-valid-and-without-any-errors",
 	}
 	invalidHostnames := map[string]string{
 		"^hostname": "invalid hostname format for --hostname: ^hostname",
@@ -397,6 +398,7 @@ func TestParseHostname(t *testing.T) {
 		"host&name": "invalid hostname format for --hostname: host&name",
 		"-hostname": "invalid hostname format for --hostname: -hostname",
 		"host_name": "invalid hostname format for --hostname: host_name",
+		"hostname-of-65-bytes-long-should-be-invalid-and-be-given-an-error": "invalid hostname format for --hostname: hostname-of-65-bytes-long-should-be-invalid-and-be-given-an-error",
 	}
 	hostnameWithDomain := "--hostname=hostname.domainname"
 	hostnameWithDomainTld := "--hostname=hostname.domainname.tld"