diff --git a/moto/ecs/models.py b/moto/ecs/models.py
index 30e4687c4..33d4dcf72 100644
--- a/moto/ecs/models.py
+++ b/moto/ecs/models.py
@@ -604,7 +604,10 @@ class EC2ContainerServiceBackend(BaseBackend):
             raise Exception("{0} is not a task_definition".format(task_definition_name))
 
     def run_task(self, cluster_str, task_definition_str, count, overrides, started_by):
-        cluster_name = cluster_str.split("/")[-1]
+        if cluster_str:
+            cluster_name = cluster_str.split("/")[-1]
+        else:
+            cluster_name = "default"
         if cluster_name in self.clusters:
             cluster = self.clusters[cluster_name]
         else:
diff --git a/tests/test_ecs/test_ecs_boto3.py b/tests/test_ecs/test_ecs_boto3.py
index 69c920192..7fd90b412 100644
--- a/tests/test_ecs/test_ecs_boto3.py
+++ b/tests/test_ecs/test_ecs_boto3.py
@@ -1122,6 +1122,71 @@ def test_run_task():
     response["tasks"][0]["stoppedReason"].should.equal("")
 
 
+@mock_ec2
+@mock_ecs
+def test_run_task_default_cluster():
+    client = boto3.client("ecs", region_name="us-east-1")
+    ec2 = boto3.resource("ec2", region_name="us-east-1")
+
+    test_cluster_name = "default"
+
+    _ = client.create_cluster(clusterName=test_cluster_name)
+
+    test_instance = ec2.create_instances(
+        ImageId="ami-1234abcd", MinCount=1, MaxCount=1
+    )[0]
+
+    instance_id_document = json.dumps(
+        ec2_utils.generate_instance_identity_document(test_instance)
+    )
+
+    response = client.register_container_instance(
+        cluster=test_cluster_name, instanceIdentityDocument=instance_id_document
+    )
+
+    _ = client.register_task_definition(
+        family="test_ecs_task",
+        containerDefinitions=[
+            {
+                "name": "hello_world",
+                "image": "docker/hello-world:latest",
+                "cpu": 1024,
+                "memory": 400,
+                "essential": True,
+                "environment": [
+                    {"name": "AWS_ACCESS_KEY_ID", "value": "SOME_ACCESS_KEY"}
+                ],
+                "logConfiguration": {"logDriver": "json-file"},
+            }
+        ],
+    )
+    response = client.run_task(
+        launchType="FARGATE",
+        overrides={},
+        taskDefinition="test_ecs_task",
+        count=2,
+        startedBy="moto",
+    )
+    len(response["tasks"]).should.equal(2)
+    response["tasks"][0]["taskArn"].should.contain(
+        "arn:aws:ecs:us-east-1:012345678910:task/"
+    )
+    response["tasks"][0]["clusterArn"].should.equal(
+        "arn:aws:ecs:us-east-1:012345678910:cluster/default"
+    )
+    response["tasks"][0]["taskDefinitionArn"].should.equal(
+        "arn:aws:ecs:us-east-1:012345678910:task-definition/test_ecs_task:1"
+    )
+    response["tasks"][0]["containerInstanceArn"].should.contain(
+        "arn:aws:ecs:us-east-1:012345678910:container-instance/"
+    )
+    response["tasks"][0]["overrides"].should.equal({})
+    response["tasks"][0]["lastStatus"].should.equal("RUNNING")
+    response["tasks"][0]["desiredStatus"].should.equal("RUNNING")
+    response["tasks"][0]["startedBy"].should.equal("moto")
+    response["tasks"][0]["stoppedReason"].should.equal("")
+
+
 @mock_ec2
 @mock_ecs
 def test_start_task():