diff --git a/moto/ecs/models.py b/moto/ecs/models.py
--- a/moto/ecs/models.py
+++ b/moto/ecs/models.py
@@ -808,7 +808,7 @@ def default_vpc_endpoint_service(service_region, zones):
             service_region, zones, "ecs"
         )
 
-    def _get_cluster(self, name):
+    def _get_cluster(self, name: str) -> Cluster:
         # short name or full ARN of the cluster
         cluster_name = name.split("/")[-1]
 
@@ -1333,10 +1333,10 @@ def create_service(
         return service
 
     def list_services(self, cluster_str, scheduling_strategy=None, launch_type=None):
-        cluster_name = cluster_str.split("/")[-1]
+        cluster = self._get_cluster(cluster_str)
         service_arns = []
         for key, service in self.services.items():
-            if cluster_name + ":" not in key:
+            if cluster.name + ":" not in key:
                 continue
 
             if (

Thanks for raising this @Bharat23 - marking it as an enhancement to implement this validation.

Just as an implementation note: the docs state that `If you do not specify a cluster, the default cluster is assumed.` Which means that the same error may be raised if you do not specify a cluster, and the `default` cluster does not exist.
diff --git a/tests/test_ecs/test_ecs_boto3.py b/tests/test_ecs/test_ecs_boto3.py
--- a/tests/test_ecs/test_ecs_boto3.py
+++ b/tests/test_ecs/test_ecs_boto3.py
@@ -701,6 +701,17 @@ def test_list_services():
     cluster1_fargate_services["serviceArns"][0].should.equal(test_ecs_service2_arn)
 
 
+@mock_ecs
+@pytest.mark.parametrize("args", [{}, {"cluster": "foo"}], ids=["no args", "unknown"])
+def test_list_unknown_service(args):
+    client = boto3.client("ecs", region_name="us-east-1")
+    with pytest.raises(ClientError) as exc:
+        client.list_services(**args)
+    err = exc.value.response["Error"]
+    err["Code"].should.equal("ClusterNotFoundException")
+    err["Message"].should.equal("Cluster not found.")
+
+
 @mock_ecs
 def test_describe_services():
     client = boto3.client("ecs", region_name="us-east-1")

ECS list_services ClusterNotFoundException Not raised
## Reporting Bugs

Resource/Client:
**ECS**

Method:
**list_services**

When calling the `ecs` `list_services` with a cluster that does not exist (in moto) the API returns an empty list response.

Expected behavior:
It should raise `ClusterNotFoundException` if cluster_name is invalid.

Moto version: 4.1.0
Python: 3.9


tests/test_ecs/test_ecs_boto3.py::test_list_unknown_service[unknown] tests/test_ecs/test_ecs_boto3.py::test_list_unknown_service[no
tests/test_ecs/test_ecs_boto3.py::test_update_missing_service tests/test_ecs/test_ecs_boto3.py::test_start_task_with_tags tests/test_ecs/test_ecs_boto3.py::test_update_service tests/test_ecs/test_ecs_boto3.py::test_list_task_definitions_with_family_prefix tests/test_ecs/test_ecs_boto3.py::test_delete_service_force tests/test_ecs/test_ecs_boto3.py::test_ecs_service_tag_resource_overwrites_tag tests/test_ecs/test_ecs_boto3.py::test_ecs_service_untag_resource tests/test_ecs/test_ecs_boto3.py::test_describe_task_definition_by_family tests/test_ecs/test_ecs_boto3.py::test_describe_services_with_known_unknown_services tests/test_ecs/test_ecs_boto3.py::test_deregister_container_instance tests/test_ecs/test_ecs_boto3.py::test_describe_clusters_missing tests/test_ecs/test_ecs_boto3.py::test_describe_task_definitions tests/test_ecs/test_ecs_boto3.py::test_run_task_awsvpc_network_error tests/test_ecs/test_ecs_boto3.py::test_delete_cluster tests/test_ecs/test_ecs_boto3.py::test_list_tasks_with_filters tests/test_ecs/test_ecs_boto3.py::test_list_tags_for_resource_ecs_service tests/test_ecs/test_ecs_boto3.py::test_update_task_set tests/test_ecs/test_ecs_boto3.py::test_register_container_instance_new_arn_format tests/test_ecs/test_ecs_boto3.py::test_list_task_definition_families tests/test_ecs/test_ecs_boto3.py::test_describe_services_error_unknown_cluster tests/test_ecs/test_ecs_boto3.py::test_describe_services tests/test_ecs/test_ecs_boto3.py::test_ecs_service_untag_resource_multiple_tags tests/test_ecs/test_ecs_boto3.py::test_list_tags_for_resource tests/test_ecs/test_ecs_boto3.py::test_create_task_set_errors tests/test_ecs/test_ecs_boto3.py::test_run_task_awsvpc_network tests/test_ecs/test_ecs_boto3.py::test_start_task tests/test_ecs/test_ecs_boto3.py::test_create_service tests/test_ecs/test_ecs_boto3.py::test_delete_service_exceptions tests/test_ecs/test_ecs_boto3.py::test_list_tasks_exceptions tests/test_ecs/test_ecs_boto3.py::test_create_service_load_balancing tests/test_ecs/test_ecs_boto3.py::test_list_container_instances tests/test_ecs/test_ecs_boto3.py::test_describe_tasks_empty_tags tests/test_ecs/test_ecs_boto3.py::test_task_definitions_unable_to_be_placed tests/test_ecs/test_ecs_boto3.py::test_deregister_task_definition_2 tests/test_ecs/test_ecs_boto3.py::test_run_task_default_cluster_new_arn_format tests/test_ecs/test_ecs_boto3.py::test_delete_service tests/test_ecs/test_ecs_boto3.py::test_create_cluster_with_setting tests/test_ecs/test_ecs_boto3.py::test_create_service_scheduling_strategy tests/test_ecs/test_ecs_boto3.py::test_list_task_definitions tests/test_ecs/test_ecs_boto3.py::test_register_container_instance tests/test_ecs/test_ecs_boto3.py::test_attributes tests/test_ecs/test_ecs_boto3.py::test_describe_container_instances tests/test_ecs/test_ecs_boto3.py::test_create_cluster tests/test_ecs/test_ecs_boto3.py::test_list_tags_exceptions tests/test_ecs/test_ecs_boto3.py::test_list_tasks tests/test_ecs/test_ecs_boto3.py::test_deregister_task_definition_1 tests/test_ecs/test_ecs_boto3.py::test_resource_reservation_and_release_memory_reservation tests/test_ecs/test_ecs_boto3.py::test_run_task_exceptions tests/test_ecs/test_ecs_boto3.py::test_task_definitions_with_port_clash tests/test_ecs/test_ecs_boto3.py::test_create_task_set tests/test_ecs/test_ecs_boto3.py::test_resource_reservation_and_release tests/test_ecs/test_ecs_boto3.py::test_ecs_service_tag_resource[enabled] tests/test_ecs/test_ecs_boto3.py::test_describe_tasks_include_tags tests/test_ecs/test_ecs_boto3.py::test_ecs_service_tag_resource[disabled] tests/test_ecs/test_ecs_boto3.py::test_list_services tests/test_ecs/test_ecs_boto3.py::test_poll_endpoint tests/test_ecs/test_ecs_boto3.py::test_update_container_instances_state tests/test_ecs/test_ecs_boto3.py::test_delete_service__using_arns tests/test_ecs/test_ecs_boto3.py::test_describe_clusters tests/test_ecs/test_ecs_boto3.py::test_stop_task tests/test_ecs/test_ecs_boto3.py::test_describe_tasks tests/test_ecs/test_ecs_boto3.py::test_update_container_instances_state_by_arn tests/test_ecs/test_ecs_boto3.py::test_stop_task_exceptions tests/test_ecs/test_ecs_boto3.py::test_ecs_task_definition_placement_constraints tests/test_ecs/test_ecs_boto3.py::test_default_container_instance_attributes tests/test_ecs/test_ecs_boto3.py::test_run_task tests/test_ecs/test_ecs_boto3.py::test_describe_services_new_arn tests/test_ecs/test_ecs_boto3.py::test_describe_services_scheduling_strategy tests/test_ecs/test_ecs_boto3.py::test_update_service_exceptions tests/test_ecs/test_ecs_boto3.py::test_start_task_exceptions tests/test_ecs/test_ecs_boto3.py::test_update_service_primary_task_set tests/test_ecs/test_ecs_boto3.py::test_delete_task_set tests/test_ecs/test_ecs_boto3.py::test_describe_container_instances_exceptions tests/test_ecs/test_ecs_boto3.py::test_describe_tasks_exceptions tests/test_ecs/test_ecs_boto3.py::test_create_service_errors tests/test_ecs/test_ecs_boto3.py::test_describe_container_instances_with_attributes tests/test_ecs/test_ecs_boto3.py::test_delete_cluster_exceptions tests/test_ecs/test_ecs_boto3.py::test_describe_task_sets tests/test_ecs/test_ecs_boto3.py::test_list_clusters tests/test_ecs/test_ecs_boto3.py::test_register_task_definition tests/test_ecs/test_ecs_boto3.py::test_run_task_default_cluster