diff --git a/moto/ecs/models.py b/moto/ecs/models.py
index 1a385226b..254f220d4 100644
--- a/moto/ecs/models.py
+++ b/moto/ecs/models.py
@@ -931,9 +931,10 @@ class EC2ContainerServiceBackend(BaseBackend):
         return result
 
     def update_service(
-        self, cluster_str, service_name, task_definition_str, desired_count
+        self, cluster_str, service_str, task_definition_str, desired_count
     ):
         cluster_name = cluster_str.split("/")[-1]
+        service_name = service_str.split("/")[-1]
         cluster_service_pair = "{0}:{1}".format(cluster_name, service_name)
         if cluster_service_pair in self.services:
             if task_definition_str is not None:
diff --git a/tests/test_ecs/test_ecs_boto3.py b/tests/test_ecs/test_ecs_boto3.py
index f6de59597..80ba304fd 100644
--- a/tests/test_ecs/test_ecs_boto3.py
+++ b/tests/test_ecs/test_ecs_boto3.py
@@ -660,6 +660,15 @@ def test_update_service():
     response["service"]["desiredCount"].should.equal(0)
     response["service"]["schedulingStrategy"].should.equal("REPLICA")
 
+    # Verify we can pass the ARNs of the cluster and service
+    response = client.update_service(
+        cluster=response["service"]["clusterArn"],
+        service=response["service"]["serviceArn"],
+        taskDefinition="test_ecs_task",
+        desiredCount=1,
+    )
+    response["service"]["desiredCount"].should.equal(1)
+
 
 @mock_ecs
 def test_update_missing_service():