diff --git a/moto/ecs/models.py b/moto/ecs/models.py
index 5689ccbdb19a..b05c3749f475 100644
--- a/moto/ecs/models.py
+++ b/moto/ecs/models.py
@@ -1138,6 +1138,14 @@ def register_task_definition(
         pid_mode: Optional[str] = None,
         ephemeral_storage: Optional[Dict[str, int]] = None,
     ) -> TaskDefinition:
+
+        if requires_compatibilities and "FARGATE" in requires_compatibilities:
+            # TODO need more validation for Fargate
+            if pid_mode and pid_mode != "task":
+                raise EcsClientException(
+                    f"Tasks using the Fargate launch type do not support pidMode '{pid_mode}'. The supported value for pidMode is 'task'."
+                )
+
         if family in self.task_definitions:
             last_id = self._get_last_task_definition_revision_id(family)
             revision = (last_id or 0) + 1
diff --git a/tests/test_ecs/test_ecs_boto3.py b/tests/test_ecs/test_ecs_boto3.py
index 51b74be22d9e..912b7be5825b 100644
--- a/tests/test_ecs/test_ecs_boto3.py
+++ b/tests/test_ecs/test_ecs_boto3.py
@@ -293,6 +293,33 @@ def test_register_task_definition():
     )
 
 
+@mock_ecs
+def test_register_task_definition_fargate_with_pid_mode():
+    client = boto3.client("ecs", region_name="us-east-1")
+    definition = dict(
+        family="test_ecs_task",
+        containerDefinitions=[
+            {"name": "hello_world", "image": "hello-world:latest", "memory": 400}
+        ],
+        requiresCompatibilities=["FARGATE"],
+        pidMode="host",
+        networkMode="awsvpc",
+        cpu="256",
+        memory="512",
+    )
+
+    with pytest.raises(ClientError) as exc:
+        client.register_task_definition(**definition)
+    ex = exc.value
+    assert ex.operation_name == "RegisterTaskDefinition"
+    assert ex.response["ResponseMetadata"]["HTTPStatusCode"] == 400
+    assert ex.response["Error"]["Code"] == "ClientException"
+    assert (
+        ex.response["Error"]["Message"]
+        == "Tasks using the Fargate launch type do not support pidMode 'host'. The supported value for pidMode is 'task'."
+    )
+
+
 @mock_ecs
 def test_list_task_definitions():
     client = boto3.client("ecs", region_name="us-east-1")