diff --git a/neutron_lib/api/definitions/__init__.py b/neutron_lib/api/definitions/__init__.py
index 79ab18afe..0a9fbfb81 100644
--- a/neutron_lib/api/definitions/__init__.py
+++ b/neutron_lib/api/definitions/__init__.py
@@ -36,6 +36,7 @@ from neutron_lib.api.definitions import network
 from neutron_lib.api.definitions import network_availability_zone
 from neutron_lib.api.definitions import network_ip_availability
 from neutron_lib.api.definitions import network_mtu
+from neutron_lib.api.definitions import network_mtu_writable
 from neutron_lib.api.definitions import pagination
 from neutron_lib.api.definitions import port
 from neutron_lib.api.definitions import port_security
@@ -80,6 +81,7 @@ _ALL_API_DEFINITIONS = {
     network_availability_zone,
     network_ip_availability,
     network_mtu,
+    network_mtu_writable,
     pagination,
     port,
     port_security,
diff --git a/neutron_lib/api/definitions/network_mtu_writable.py b/neutron_lib/api/definitions/network_mtu_writable.py
new file mode 100644
index 000000000..b603a7f85
--- /dev/null
+++ b/neutron_lib/api/definitions/network_mtu_writable.py
@@ -0,0 +1,82 @@
+# 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 import converters
+from neutron_lib.api.definitions import network
+from neutron_lib.api.definitions import network_mtu
+
+
+MTU = 'mtu'
+
+# The alias of the extension.
+ALIAS = 'net-mtu-writable'
+
+# The label to lookup the plugin in the plugin directory. It can match the
+# alias, as required.
+LABEL = ALIAS
+
+# 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 = 'Network MTU (writable)'
+
+# A prefix for API resources. An empty prefix means that the API is going
+# to be exposed at the v2/ level as any other core resource.
+API_PREFIX = ''
+
+# The description of the extension.
+DESCRIPTION = (
+    "Provides a writable MTU attribute for a network resource.")
+
+# A timestamp of when the extension was introduced.
+UPDATED_TIMESTAMP = "2017-07-12T00:00:00-00:00"
+
+
+RESOURCE_ATTRIBUTE_MAP = {
+    network.COLLECTION_NAME: {
+        MTU: {'allow_post': True, 'allow_put': True, 'is_visible': True,
+              'validate': {'type:non_negative': None}, 'default': 0,
+              'convert_to': converters.convert_to_int},
+    },
+}
+
+# The subresource attribute map for the extension. It adds child resources
+# to main extension's resource. The subresource map must have a parent and
+# a parameters entry. If an extension does not need such a map, None can
+# be specified (mandatory).
+SUB_RESOURCE_ATTRIBUTE_MAP = {}
+
+# The action map: it associates verbs with methods to be performed on
+# the API resource.
+ACTION_MAP = {}
+
+# The action status.
+ACTION_STATUS = {
+}
+
+# The list of required extensions.
+REQUIRED_EXTENSIONS = [network_mtu.ALIAS]
+
+# The list of optional extensions.
+OPTIONAL_EXTENSIONS = []
diff --git a/neutron_lib/tests/unit/api/definitions/test_network_mtu_writable.py b/neutron_lib/tests/unit/api/definitions/test_network_mtu_writable.py
new file mode 100644
index 000000000..dc9871f52
--- /dev/null
+++ b/neutron_lib/tests/unit/api/definitions/test_network_mtu_writable.py
@@ -0,0 +1,19 @@
+#    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 network_mtu_writable
+from neutron_lib.tests.unit.api.definitions import base
+
+
+class NetworkMtuDefinitionTestCase(base.DefinitionBaseTestCase):
+    extension_module = network_mtu_writable
+    extension_attributes = ('mtu',)
diff --git a/releasenotes/notes/create-netmtu-writable-extension-284892119ef6595c.yaml b/releasenotes/notes/create-netmtu-writable-extension-284892119ef6595c.yaml
new file mode 100644
index 000000000..b429ead35
--- /dev/null
+++ b/releasenotes/notes/create-netmtu-writable-extension-284892119ef6595c.yaml
@@ -0,0 +1,6 @@
+---
+features:
+  - The new ``net-mtu-writable`` extension API definition has been added as
+    ``neutron_lib.api.definitions.network_mtu_writable``. The new extension
+    indicates that the network ``mtu`` attribute is writeable.  Plugins
+    supporting the new extension are expected to also support ``net-mtu``.