diff --git a/neutron_lib/api/definitions/__init__.py b/neutron_lib/api/definitions/__init__.py
index 144dab4aa..7d4f8f6de 100644
--- a/neutron_lib/api/definitions/__init__.py
+++ b/neutron_lib/api/definitions/__init__.py
@@ -66,6 +66,7 @@ from neutron_lib.api.definitions import port_mac_address_regenerate
 from neutron_lib.api.definitions import port_security
 from neutron_lib.api.definitions import portbindings
 from neutron_lib.api.definitions import portbindings_extended
+from neutron_lib.api.definitions import project_default_networks
 from neutron_lib.api.definitions import project_id
 from neutron_lib.api.definitions import provider_net
 from neutron_lib.api.definitions import qos
@@ -155,6 +156,7 @@ _ALL_API_DEFINITIONS = {
     port_security,
     portbindings,
     portbindings_extended,
+    project_default_networks,
     project_id,
     provider_net,
     qos,
diff --git a/neutron_lib/api/definitions/base.py b/neutron_lib/api/definitions/base.py
index edddd7c46..fa7a04907 100644
--- a/neutron_lib/api/definitions/base.py
+++ b/neutron_lib/api/definitions/base.py
@@ -81,6 +81,7 @@ KNOWN_EXTENSIONS = (
     'availability_zone',
     'binding',
     'data-plane-status',
+    'project-default-networks',
     'default-subnetpools',
     'dhcp_agent_scheduler',
     'dns-domain-ports',
diff --git a/neutron_lib/api/definitions/project_default_networks.py b/neutron_lib/api/definitions/project_default_networks.py
new file mode 100644
index 000000000..75888bd97
--- /dev/null
+++ b/neutron_lib/api/definitions/project_default_networks.py
@@ -0,0 +1,79 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from neutron_lib.api import converters
+from neutron_lib.api.definitions import network
+
+
+PROJECT_DEFAULT = 'project_default'
+
+# The alias of the extension.
+ALIAS = 'project-default-networks'
+
+# Whether or not this extension is simply signaling behavior to the user
+# or it actively modifies the attribute map.
+IS_SHIM_EXTENSION = False
+
+# Whether the extension is marking the adoption of standardattr model for
+# legacy resources, or introducing new standardattr attributes. False or
+# None if the standardattr model is adopted since the introduction of
+# resource extension.
+# If this is True, the alias for the extension should be prefixed with
+# 'standard-attr-'.
+IS_STANDARD_ATTR_EXTENSION = False
+
+# The name of the extension.
+NAME = 'Default network for project'
+
+# The description of the extension.
+DESCRIPTION = "Support specifying networks as default networks for projects"
+
+# A timestamp of when the extension was introduced.
+UPDATED_TIMESTAMP = "2018-07-03T10:00:00-00:00"
+
+# The name of the resource introduced or being extended.
+RESOURCE_NAME = network.RESOURCE_NAME
+
+# The plural for the resource introduced or being extended.
+COLLECTION_NAME = network.COLLECTION_NAME
+
+# The resource attribute map for the extension.
+RESOURCE_ATTRIBUTE_MAP = {
+    COLLECTION_NAME: {
+        PROJECT_DEFAULT: {
+            'allow_post': True, 'allow_put': True,
+            'default': False,
+            'convert_to': converters.convert_to_boolean,
+            'is_visible': True
+        }
+    }
+}
+
+# The subresource attribute map for the extension.
+SUB_RESOURCE_ATTRIBUTE_MAP = {
+}
+
+# The action map.
+ACTION_MAP = {
+}
+
+# The action status.
+ACTION_STATUS = {
+}
+
+# The list of required extensions.
+REQUIRED_EXTENSIONS = [
+]
+
+# The list of optional extensions.
+OPTIONAL_EXTENSIONS = [
+]
diff --git a/neutron_lib/tests/unit/api/definitions/test_project_default_networks.py b/neutron_lib/tests/unit/api/definitions/test_project_default_networks.py
new file mode 100644
index 000000000..de3ac62a7
--- /dev/null
+++ b/neutron_lib/tests/unit/api/definitions/test_project_default_networks.py
@@ -0,0 +1,21 @@
+# Copyright (c) 2017 NEC Corporation.  All rights reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from neutron_lib.api.definitions import project_default_networks as dn
+from neutron_lib.tests.unit.api.definitions import base
+
+
+class DefaultNetworksDefinitionTestCase(base.DefinitionBaseTestCase):
+    extension_module = dn
+    extension_attributes = (dn.PROJECT_DEFAULT,)
diff --git a/releasenotes/notes/add-is-default-to-network-d16a2e6bcfae943a.yaml b/releasenotes/notes/add-is-default-to-network-d16a2e6bcfae943a.yaml
new file mode 100644
index 000000000..2456a118f
--- /dev/null
+++ b/releasenotes/notes/add-is-default-to-network-d16a2e6bcfae943a.yaml
@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    The ``project-default-networks`` extension is now available and adds a new
+    attribute ``project_default`` into the ``network`` resource.
+    This attribute will be used to indicate if a network is a project
+    default network.