diff --git a/api/types/kubernetes_test.go b/api/types/kubernetes_test.go
index 5d3a81210be..1763da9e114 100644
--- a/api/types/kubernetes_test.go
+++ b/api/types/kubernetes_test.go
@@ -30,7 +30,7 @@ func TestKubeClustersSorter(t *testing.T) {
 		servers := make([]KubeCluster, len(testVals))
 		for i := 0; i < len(testVals); i++ {
 			var err error
-			servers[i], err = NewKubernetesClusterV3FromLegacyCluster("_", &KubernetesCluster{
+			servers[i], err = NewKubernetesClusterV3FromLegacyCluster("", &KubernetesCluster{
 				Name: testVals[i],
 			})
 			require.NoError(t, err)
diff --git a/api/types/namespace.go b/api/types/namespace.go
index 41e4b3e4fe7..a6fbd02de22 100644
--- a/api/types/namespace.go
+++ b/api/types/namespace.go
@@ -143,3 +143,17 @@ func IsValidNamespace(s string) bool {
 }
 
 var validNamespace = regexp.MustCompile(`^[A-Za-z0-9]+$`)
+
+// ValidateNamespaceDefault ensures that the namespace is the "default"
+// namespace.
+// This is a precursor to a hard-removal of namespaces.
+func ValidateNamespaceDefault(ns string) error {
+	if ns == defaults.Namespace {
+		return nil
+	}
+
+	const message = "" +
+		"namespace %q invalid, custom namespaces are deprecated; " +
+		"the namespace field should be omitted or set to %q"
+	return trace.BadParameter(message, ns, defaults.Namespace)
+}
diff --git a/api/types/presence.go b/api/types/presence.go
index 764f4671998..b27b3b232cc 100644
--- a/api/types/presence.go
+++ b/api/types/presence.go
@@ -67,9 +67,13 @@ func (s *KeepAlive) CheckAndSetDefaults() error {
 	if s.IsEmpty() {
 		return trace.BadParameter("missing resource name")
 	}
+
 	if s.Namespace == "" {
 		s.Namespace = defaults.Namespace
 	}
+	if err := ValidateNamespaceDefault(s.Namespace); err != nil {
+		return trace.Wrap(err)
+	}
 
 	return nil
 }
diff --git a/api/types/resource.go b/api/types/resource.go
index f7465a84e8b..ad5beaceb78 100644
--- a/api/types/resource.go
+++ b/api/types/resource.go
@@ -478,9 +478,13 @@ func (m *Metadata) CheckAndSetDefaults() error {
 	if m.Name == "" {
 		return trace.BadParameter("missing parameter Name")
 	}
+
 	if m.Namespace == "" {
 		m.Namespace = defaults.Namespace
 	}
+	if err := ValidateNamespaceDefault(m.Namespace); err != nil {
+		return trace.Wrap(err)
+	}
 
 	// adjust expires time to UTC if it's set
 	if m.Expires != nil {
diff --git a/api/types/resource_test.go b/api/types/resource_test.go
index 896ee3b78ec..53b38ef33e1 100644
--- a/api/types/resource_test.go
+++ b/api/types/resource_test.go
@@ -266,7 +266,7 @@ func TestMatchSearch_ResourceSpecific(t *testing.T) {
 			name:               "kube cluster",
 			matchingSearchVals: []string{"foo", "prod", "env"},
 			newResource: func(t *testing.T) ResourceWithLabels {
-				kc, err := NewKubernetesClusterV3FromLegacyCluster("_", &KubernetesCluster{
+				kc, err := NewKubernetesClusterV3FromLegacyCluster("", &KubernetesCluster{
 					Name:         "foo",
 					StaticLabels: labels,
 				})
diff --git a/api/types/role.go b/api/types/role.go
index d95562d5d6f..6551921bc9a 100644
--- a/api/types/role.go
+++ b/api/types/role.go
@@ -1071,8 +1071,9 @@ func (r *RoleV6) CheckAndSetDefaults() error {
 	if len(r.Spec.Options.BPF) == 0 {
 		r.Spec.Options.BPF = defaults.EnhancedEvents()
 	}
-	if r.Spec.Allow.Namespaces == nil {
-		r.Spec.Allow.Namespaces = []string{defaults.Namespace}
+	if err := checkAndSetRoleConditionNamespaces(&r.Spec.Allow.Namespaces); err != nil {
+		// Using trace.BadParameter instead of trace.Wrap for a better error message.
+		return trace.BadParameter("allow: %s", err)
 	}
 	if r.Spec.Options.RecordSession == nil {
 		r.Spec.Options.RecordSession = &RecordSession{
@@ -1175,8 +1176,9 @@ func (r *RoleV6) CheckAndSetDefaults() error {
 		return trace.BadParameter("unrecognized role version: %v", r.Version)
 	}
 
-	if r.Spec.Deny.Namespaces == nil {
-		r.Spec.Deny.Namespaces = []string{defaults.Namespace}
+	if err := checkAndSetRoleConditionNamespaces(&r.Spec.Deny.Namespaces); err != nil {
+		// Using trace.BadParameter instead of trace.Wrap for a better error message.
+		return trace.BadParameter("deny: %s", err)
 	}
 
 	// Validate request.kubernetes_resources fields are all valid.
@@ -1322,6 +1324,27 @@ func (r *RoleV6) CheckAndSetDefaults() error {
 	return nil
 }
 
+func checkAndSetRoleConditionNamespaces(namespaces *[]string) error {
+	// If nil use the default.
+	// This distinguishes between nil and empty (in accordance to legacy code).
+	if *namespaces == nil {
+		*namespaces = []string{defaults.Namespace}
+		return nil
+	}
+
+	for i, ns := range *namespaces {
+		if ns == Wildcard {
+			continue // OK, wildcard is accepted.
+		}
+		if err := ValidateNamespaceDefault(ns); err != nil {
+			// Using trace.BadParameter instead of trace.Wrap for a better error message.
+			return trace.BadParameter("namespaces[%d]: %s", i, err)
+		}
+	}
+
+	return nil
+}
+
 // String returns the human readable representation of a role.
 func (r *RoleV6) String() string {
 	options, _ := json.Marshal(r.Spec.Options)
diff --git a/lib/services/local/databaseservice.go b/lib/services/local/databaseservice.go
index ad7f064e35b..255f777eba6 100644
--- a/lib/services/local/databaseservice.go
+++ b/lib/services/local/databaseservice.go
@@ -44,6 +44,10 @@ func (s *DatabaseServicesService) UpsertDatabaseService(ctx context.Context, ser
 	if err := services.CheckAndSetDefaults(service); err != nil {
 		return nil, trace.Wrap(err)
 	}
+	if err := types.ValidateNamespaceDefault(service.GetNamespace()); err != nil {
+		return nil, trace.Wrap(err)
+	}
+
 	rev := service.GetRevision()
 	value, err := services.MarshalDatabaseService(service)
 	if err != nil {
diff --git a/lib/services/local/presence.go b/lib/services/local/presence.go
index e7424e500f0..19b391029cd 100644
--- a/lib/services/local/presence.go
+++ b/lib/services/local/presence.go
@@ -345,10 +345,10 @@ func (s *PresenceService) UpsertNode(ctx context.Context, server types.Server) (
 	if server.GetNamespace() == "" {
 		server.SetNamespace(apidefaults.Namespace)
 	}
-
-	if n := server.GetNamespace(); n != apidefaults.Namespace {
-		return nil, trace.BadParameter("cannot place node in namespace %q, custom namespaces are deprecated", n)
+	if err := types.ValidateNamespaceDefault(server.GetNamespace()); err != nil {
+		return nil, trace.Wrap(err)
 	}
+
 	rev := server.GetRevision()
 	value, err := services.MarshalServer(server)
 	if err != nil {
@@ -377,10 +377,10 @@ func (s *PresenceService) UpdateNode(ctx context.Context, server types.Server) (
 	if server.GetNamespace() == "" {
 		server.SetNamespace(apidefaults.Namespace)
 	}
-
-	if n := server.GetNamespace(); n != apidefaults.Namespace {
-		return nil, trace.BadParameter("cannot place node in namespace %q, custom namespaces are deprecated", n)
+	if err := types.ValidateNamespaceDefault(server.GetNamespace()); err != nil {
+		return nil, trace.Wrap(err)
 	}
+
 	rev := server.GetRevision()
 	value, err := services.MarshalServer(server)
 	if err != nil {
@@ -1003,6 +1003,10 @@ func (s *PresenceService) UpsertDatabaseServer(ctx context.Context, server types
 	if err := services.CheckAndSetDefaults(server); err != nil {
 		return nil, trace.Wrap(err)
 	}
+	if err := types.ValidateNamespaceDefault(server.GetNamespace()); err != nil {
+		return nil, trace.Wrap(err)
+	}
+
 	rev := server.GetRevision()
 	value, err := services.MarshalDatabaseServer(server)
 	if err != nil {
@@ -1096,6 +1100,10 @@ func (s *PresenceService) UpsertApplicationServer(ctx context.Context, server ty
 	if err := services.CheckAndSetDefaults(server); err != nil {
 		return nil, trace.Wrap(err)
 	}
+	if err := types.ValidateNamespaceDefault(server.GetNamespace()); err != nil {
+		return nil, trace.Wrap(err)
+	}
+
 	rev := server.GetRevision()
 	value, err := services.MarshalAppServer(server)
 	if err != nil {
diff --git a/lib/services/matchers_test.go b/lib/services/matchers_test.go
index 68c52858dde..7238c099d60 100644
--- a/lib/services/matchers_test.go
+++ b/lib/services/matchers_test.go
@@ -499,7 +499,7 @@ func TestMatchResourceByFilters(t *testing.T) {
 		{
 			name: "kube cluster",
 			resource: func() types.ResourceWithLabels {
-				cluster, err := types.NewKubernetesClusterV3FromLegacyCluster("_", &types.KubernetesCluster{
+				cluster, err := types.NewKubernetesClusterV3FromLegacyCluster("", &types.KubernetesCluster{
 					Name: "foo",
 				})
 				require.NoError(t, err)