diff --git a/CHANGELOG.md b/CHANGELOG.md
index a4e249459..c9e20de71 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -32,6 +32,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Running a container which does not have its own mount namespace no longer
   changes the propagation of host's `/` and of the parent mount of the
   container rootfs (they were made private or slave). (#5537)
+- runc now errors out when seccomp `defaultErrnoRet` or `errnoRet` is set for
+  an action which does not support it (i.e. anything other than
+  `SCMP_ACT_ERRNO` and `SCMP_ACT_TRACE`), as required by the runtime spec.
+  Previously, such values were silently ignored. (#5550, #5564)
 
 ## [1.6.0-rc.1] - 2026-10-06
 
diff --git a/libcontainer/configs/validate/validator.go b/libcontainer/configs/validate/validator.go
index 5960661db..df046dd2a 100644
--- a/libcontainer/configs/validate/validator.go
+++ b/libcontainer/configs/validate/validator.go
@@ -32,6 +32,7 @@ func Validate(config *configs.Config) error {
 		rootfs,
 		rootlessEUIDCheck,
 		scheduler,
+		seccompCheck,
 		selinuxCheck,
 		sysctl,
 		uts,
@@ -543,3 +544,24 @@ func memoryPolicy(config *configs.Config) error {
 	}
 	return nil
 }
+
+func seccompCheck(config *configs.Config) error {
+	sc := config.Seccomp
+	if sc == nil {
+		return nil
+	}
+	// The runtime spec requires an error if errnoRet
+	// is set for an action that does not support it.
+	supportsErrno := func(a configs.Action) bool {
+		return a == configs.Errno || a == configs.Trace
+	}
+	if sc.DefaultErrnoRet != nil && !supportsErrno(sc.DefaultAction) {
+		return errors.New("seccomp: defaultErrnoRet is only supported for SCMP_ACT_ERRNO and SCMP_ACT_TRACE actions")
+	}
+	for _, call := range sc.Syscalls {
+		if call != nil && call.ErrnoRet != nil && !supportsErrno(call.Action) {
+			return fmt.Errorf("seccomp: syscall %q: errnoRet is only supported for SCMP_ACT_ERRNO and SCMP_ACT_TRACE actions", call.Name)
+		}
+	}
+	return nil
+}
diff --git a/libcontainer/configs/validate/validator_test.go b/libcontainer/configs/validate/validator_test.go
index fd7a26337..b0bf75ba6 100644
--- a/libcontainer/configs/validate/validator_test.go
+++ b/libcontainer/configs/validate/validator_test.go
@@ -1168,3 +1168,80 @@ func TestDevValidName(t *testing.T) {
 		})
 	}
 }
+
+func TestValidateSeccompErrnoRet(t *testing.T) {
+	errno := uint(1)
+	testCases := []struct {
+		name    string
+		seccomp *configs.Seccomp
+		isErr   bool
+	}{
+		{
+			name: "defaultErrnoRet with SCMP_ACT_ERRNO",
+			seccomp: &configs.Seccomp{
+				DefaultAction:   configs.Errno,
+				DefaultErrnoRet: &errno,
+			},
+		},
+		{
+			name: "defaultErrnoRet with SCMP_ACT_TRACE",
+			seccomp: &configs.Seccomp{
+				DefaultAction:   configs.Trace,
+				DefaultErrnoRet: &errno,
+			},
+		},
+		{
+			name: "defaultErrnoRet with SCMP_ACT_ALLOW",
+			seccomp: &configs.Seccomp{
+				DefaultAction:   configs.Allow,
+				DefaultErrnoRet: &errno,
+			},
+			isErr: true,
+		},
+		{
+			name: "errnoRet with SCMP_ACT_ERRNO",
+			seccomp: &configs.Seccomp{
+				DefaultAction: configs.Allow,
+				Syscalls: []*configs.Syscall{
+					{Name: "mkdir", Action: configs.Errno, ErrnoRet: &errno},
+				},
+			},
+		},
+		{
+			name: "errnoRet with SCMP_ACT_KILL_PROCESS",
+			seccomp: &configs.Seccomp{
+				DefaultAction: configs.Allow,
+				Syscalls: []*configs.Syscall{
+					{Name: "mkdir", Action: configs.KillProcess, ErrnoRet: &errno},
+				},
+			},
+			isErr: true,
+		},
+		{
+			name: "errnoRet with redundant SCMP_ACT_ALLOW",
+			seccomp: &configs.Seccomp{
+				DefaultAction: configs.Allow,
+				Syscalls: []*configs.Syscall{
+					{Name: "mkdir", Action: configs.Allow, ErrnoRet: &errno},
+				},
+			},
+			isErr: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			config := &configs.Config{
+				Rootfs:  "/var",
+				Seccomp: tc.seccomp,
+			}
+			err := Validate(config)
+			if tc.isErr && err == nil {
+				t.Error("expected error, got nil")
+			}
+			if !tc.isErr && err != nil {
+				t.Errorf("expected nil, got error %v", err)
+			}
+		})
+	}
+}
diff --git a/tests/integration/seccomp.bats b/tests/integration/seccomp.bats
index 87dc8aa23..414dd80c9 100644
--- a/tests/integration/seccomp.bats
+++ b/tests/integration/seccomp.bats
@@ -60,6 +60,27 @@ function teardown() {
 	assert_output --partial "Network is down"
 }
 
+@test "runc run [seccomp] (errnoRet with unsupported action)" {
+	update_config '.process.args = ["/bin/true"]'
+
+	# defaultErrnoRet with a default action not supporting it.
+	update_config '.linux.seccomp = {
+				"defaultAction":"SCMP_ACT_ALLOW",
+				"defaultErrnoRet": 1
+			}'
+	run ! runc run test_busybox
+	assert_output --partial "defaultErrnoRet is only supported for"
+
+	# errnoRet with a syscall action not supporting it (the rule is
+	# redundant as it is the same as the default action).
+	update_config '.linux.seccomp = {
+				"defaultAction":"SCMP_ACT_ALLOW",
+				"syscalls":[{"names":["mkdir","mkdirat"], "action":"SCMP_ACT_ALLOW", "errnoRet": 1}]
+			}'
+	run ! runc run test_busybox
+	assert_output --regexp 'syscall .*mkdir.*: errnoRet is only supported for'
+}
+
 # Prints the numeric value of provided seccomp flags combination.
 # The parameter is flags string, as supplied in OCI spec, for example
 # '"SECCOMP_FILTER_FLAG_TSYNC","SECCOMP_FILTER_FLAG_LOG"'.