diff --git a/moto/ecs/models.py b/moto/ecs/models.py
index 845bdf650..30e4687c4 100644
--- a/moto/ecs/models.py
+++ b/moto/ecs/models.py
@@ -118,6 +118,7 @@ class TaskDefinition(BaseObject):
         revision,
         container_definitions,
         region_name,
+        network_mode=None,
         volumes=None,
         tags=None,
     ):
@@ -132,6 +133,10 @@ class TaskDefinition(BaseObject):
             self.volumes = []
         else:
             self.volumes = volumes
+        if network_mode is None:
+            self.network_mode = "bridge"
+        else:
+            self.network_mode = network_mode
 
     @property
     def response_object(self):
@@ -553,7 +558,7 @@ class EC2ContainerServiceBackend(BaseBackend):
             raise Exception("{0} is not a cluster".format(cluster_name))
 
     def register_task_definition(
-        self, family, container_definitions, volumes, tags=None
+        self, family, container_definitions, volumes=None, network_mode=None, tags=None
     ):
         if family in self.task_definitions:
             last_id = self._get_last_task_definition_revision_id(family)
@@ -562,7 +567,13 @@ class EC2ContainerServiceBackend(BaseBackend):
             self.task_definitions[family] = {}
             revision = 1
         task_definition = TaskDefinition(
-            family, revision, container_definitions, self.region_name, volumes, tags
+            family,
+            revision,
+            container_definitions,
+            self.region_name,
+            volumes=volumes,
+            network_mode=network_mode,
+            tags=tags,
         )
         self.task_definitions[family][revision] = task_definition
 
diff --git a/moto/ecs/responses.py b/moto/ecs/responses.py
index d08bded2c..49bf022b4 100644
--- a/moto/ecs/responses.py
+++ b/moto/ecs/responses.py
@@ -62,8 +62,13 @@ class EC2ContainerServiceResponse(BaseResponse):
         container_definitions = self._get_param("containerDefinitions")
         volumes = self._get_param("volumes")
         tags = self._get_param("tags")
+        network_mode = self._get_param("networkMode")
         task_definition = self.ecs_backend.register_task_definition(
-            family, container_definitions, volumes, tags
+            family,
+            container_definitions,
+            volumes=volumes,
+            network_mode=network_mode,
+            tags=tags,
         )
         return json.dumps({"taskDefinition": task_definition.response_object})
 
diff --git a/tests/test_ecs/test_ecs_boto3.py b/tests/test_ecs/test_ecs_boto3.py
index 973c95b81..f1f1e04ae 100644
--- a/tests/test_ecs/test_ecs_boto3.py
+++ b/tests/test_ecs/test_ecs_boto3.py
@@ -94,6 +94,7 @@ def test_register_task_definition():
                 "logConfiguration": {"logDriver": "json-file"},
             }
         ],
+        networkMode="bridge",
         tags=[
             {"key": "createdBy", "value": "moto-unittest"},
             {"key": "foo", "value": "bar"},
@@ -124,6 +125,7 @@ def test_register_task_definition():
     response["taskDefinition"]["containerDefinitions"][0]["logConfiguration"][
         "logDriver"
     ].should.equal("json-file")
+    response["taskDefinition"]["networkMode"].should.equal("bridge")
 
 
 @mock_ecs