diff --git a/neutron_lib/exceptions/l3.py b/neutron_lib/exceptions/l3.py
new file mode 100644
index 000000000..55f3a4320
--- /dev/null
+++ b/neutron_lib/exceptions/l3.py
@@ -0,0 +1,73 @@
+# 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._i18n import _
+from neutron_lib import exceptions
+
+
+# L3 Exceptions
+class RouterNotFound(exceptions.NotFound):
+    message = _("Router %(router_id)s could not be found")
+
+
+class RouterInUse(exceptions.InUse):
+    message = _("Router %(router_id)s %(reason)s")
+
+    def __init__(self, **kwargs):
+        if 'reason' not in kwargs:
+            kwargs['reason'] = "still has ports"
+        super(RouterInUse, self).__init__(**kwargs)
+
+
+class RouterInterfaceNotFound(exceptions.NotFound):
+    message = _("Router %(router_id)s does not have "
+                "an interface with id %(port_id)s")
+
+
+class RouterInterfaceNotFoundForSubnet(exceptions.NotFound):
+    message = _("Router %(router_id)s has no interface "
+                "on subnet %(subnet_id)s")
+
+
+class RouterInterfaceInUseByFloatingIP(exceptions.InUse):
+    message = _("Router interface for subnet %(subnet_id)s on router "
+                "%(router_id)s cannot be deleted, as it is required "
+                "by one or more floating IPs.")
+
+
+class FloatingIPNotFound(exceptions.NotFound):
+    message = _("Floating IP %(floatingip_id)s could not be found")
+
+
+class ExternalGatewayForFloatingIPNotFound(exceptions.NotFound):
+    message = _("External network %(external_network_id)s is not reachable "
+                "from subnet %(subnet_id)s.  Therefore, cannot associate "
+                "Port %(port_id)s with a Floating IP.")
+
+
+class FloatingIPPortAlreadyAssociated(exceptions.InUse):
+    message = _("Cannot associate floating IP %(floating_ip_address)s "
+                "(%(fip_id)s) with port %(port_id)s "
+                "using fixed IP %(fixed_ip)s, as that fixed IP already "
+                "has a floating IP on external network %(net_id)s.")
+
+
+class RouterExternalGatewayInUseByFloatingIp(exceptions.InUse):
+    message = _("Gateway cannot be updated for router %(router_id)s, since a "
+                "gateway to external network %(net_id)s is required by one or "
+                "more floating IPs.")
+
+
+class RouterInterfaceAttachmentConflict(exceptions.Conflict):
+    message = _("Error %(reason)s while attempting the operation.")
diff --git a/releasenotes/notes/l3-apidef-exceptions-ee57b9df1c7443d4.yaml b/releasenotes/notes/l3-apidef-exceptions-ee57b9df1c7443d4.yaml
new file mode 100644
index 000000000..97b822a31
--- /dev/null
+++ b/releasenotes/notes/l3-apidef-exceptions-ee57b9df1c7443d4.yaml
@@ -0,0 +1,4 @@
+---
+features:
+  - The exception classes from ``neutron.extensions.l3`` have
+    been rehomed into ``neutron_lib.exceptions.l3``.