diff --git a/nova/api/openstack/compute/floating_ips.py b/nova/api/openstack/compute/floating_ips.py
index e79bb46221bc..a5862e2b9d9d 100644
--- a/nova/api/openstack/compute/floating_ips.py
+++ b/nova/api/openstack/compute/floating_ips.py
@@ -267,6 +267,8 @@ class FloatingIPActionController(wsgi.Controller):
         except exception.FloatingIpAssociated:
             msg = _('floating IP is already associated')
             raise webob.exc.HTTPBadRequest(explanation=msg)
+        except exception.FloatingIpAssociateFailed as e:
+            raise webob.exc.HTTPBadRequest(explanation=e.format_message())
         except exception.NoFloatingIpInterface:
             msg = _('l3driver call to add floating IP failed')
             raise webob.exc.HTTPBadRequest(explanation=msg)
diff --git a/nova/network/neutronv2/api.py b/nova/network/neutronv2/api.py
index 5984806c5afe..42d916a2bc49 100644
--- a/nova/network/neutronv2/api.py
+++ b/nova/network/neutronv2/api.py
@@ -1730,7 +1730,10 @@ class API(base_api.NetworkAPI):
         fip = self._get_floating_ip_by_address(client, floating_address)
         param = {'port_id': port_id,
                  'fixed_ip_address': fixed_address}
-        client.update_floatingip(fip['id'], {'floatingip': param})
+        try:
+            client.update_floatingip(fip['id'], {'floatingip': param})
+        except neutron_client_exc.Conflict as e:
+            raise exception.FloatingIpAssociateFailed(six.text_type(e))
 
         if fip['port_id']:
             port = self._show_port(context, fip['port_id'],
diff --git a/nova/tests/unit/api/openstack/compute/test_floating_ips.py b/nova/tests/unit/api/openstack/compute/test_floating_ips.py
index 1cb214142dd2..0fc45b6f7287 100644
--- a/nova/tests/unit/api/openstack/compute/test_floating_ips.py
+++ b/nova/tests/unit/api/openstack/compute/test_floating_ips.py
@@ -634,6 +634,15 @@ class FloatingIpTestV21(test.TestCase):
                           self.manager._add_floating_ip, self.fake_req,
                           TEST_INST, body=body)
 
+    @mock.patch.object(network.api.API, 'associate_floating_ip',
+                       side_effect=exception.FloatingIpAssociateFailed(
+                           address='10.10.10.11'))
+    def test_associate_floating_ip_failed(self, associate_mock):
+        body = dict(addFloatingIp=dict(address='10.10.10.11'))
+        self.assertRaises(webob.exc.HTTPBadRequest,
+                          self.manager._add_floating_ip, self.fake_req,
+                          TEST_INST, body=body)
+
     def test_associate_floating_ip_bad_address_key(self):
         body = dict(addFloatingIp=dict(bad_address='10.10.10.11'))
         req = fakes.HTTPRequest.blank('/v2/fake/servers/test_inst/action')
diff --git a/nova/tests/unit/network/test_neutronv2.py b/nova/tests/unit/network/test_neutronv2.py
index 18bb9c0aa224..9f99638bd8ab 100644
--- a/nova/tests/unit/network/test_neutronv2.py
+++ b/nova/tests/unit/network/test_neutronv2.py
@@ -4783,6 +4783,36 @@ class TestNeutronv2WithMock(test.TestCase):
             self.context, pci_requests, requested_networks)
         self.assertFalse(getclient.called)
 
+    @mock.patch.object(neutronapi, 'get_client')
+    def test_associate_floating_ip_conflict(self, mock_get_client):
+        """Tests that if Neutron raises a Conflict we handle it and re-raise
+        as a nova-specific exception.
+        """
+        mock_get_client.return_value.update_floatingip.side_effect = (
+            exceptions.Conflict(
+                "Cannot associate floating IP 172.24.5.15 "
+                "(60a8f00b-4404-4518-ad66-00448a155904) with port "
+                "95ee1ffb-6d41-447d-a90e-b6ce5d9c92fa using fixed IP "
+                "10.1.0.9, as that fixed IP already has a floating IP on "
+                "external network bdcda645-f612-40ab-a956-0d95af42cf7c.")
+        )
+        with test.nested(
+            mock.patch.object(
+                self.api, '_get_port_id_by_fixed_address',
+                return_value='95ee1ffb-6d41-447d-a90e-b6ce5d9c92fa'),
+            mock.patch.object(
+                self.api, '_get_floating_ip_by_address',
+                return_value={'id': uuids.floating_ip_id})
+        ) as (
+            _get_floating_ip_by_address, _get_port_id_by_fixed_address
+        ):
+            instance = fake_instance.fake_instance_obj(
+                self.context, uuid='2a2200ec-02fe-484e-885b-9bae7b21ecba')
+            self.assertRaises(exception.FloatingIpAssociateFailed,
+                              self.api.associate_floating_ip,
+                              self.context, instance,
+                              '172.24.5.15', '10.1.0.9')
+
 
 class TestNeutronv2ModuleMethods(test.NoDBTestCase):