diff --git a/nova/cmd/manage.py b/nova/cmd/manage.py
index 40861379c42f..5bea13d5d13b 100644
--- a/nova/cmd/manage.py
+++ b/nova/cmd/manage.py
@@ -668,6 +668,11 @@ class DbCommands(object):
         quotas_obj.migrate_quota_limits_to_api_db,
         # Added in Pike
         quotas_obj.migrate_quota_classes_to_api_db,
+        # Added in Rocky
+        # NOTE(tssurya): This online migration is going to be backported to
+        # Queens and Pike since instance.avz of instances before Pike
+        # need to be populated if it was not specified during boot time.
+        instance_obj.populate_missing_availability_zones,
     )
 
     def __init__(self):
diff --git a/nova/objects/instance.py b/nova/objects/instance.py
index e175eaf25009..c0f0cd5062f8 100644
--- a/nova/objects/instance.py
+++ b/nova/objects/instance.py
@@ -25,6 +25,7 @@ from sqlalchemy.orm import joinedload
 from sqlalchemy.sql import func
 from sqlalchemy.sql import null
 
+from nova import availability_zones as avail_zone
 from nova.cells import opts as cells_opts
 from nova.cells import rpcapi as cells_rpcapi
 from nova.cells import utils as cells_utils
@@ -1204,6 +1205,20 @@ def _make_instance_list(context, inst_list, db_inst_list, expected_attrs):
     return inst_list
 
 
+@db_api.pick_context_manager_writer
+def populate_missing_availability_zones(context, count):
+    instances = (context.session.query(models.Instance).
+        filter_by(availability_zone=None).limit(count).all())
+    count_all = len(instances)
+    count_hit = 0
+    for instance in instances:
+        az = avail_zone.get_instance_availability_zone(context, instance)
+        instance.availability_zone = az
+        instance.save(context.session)
+        count_hit += 1
+    return count_all, count_hit
+
+
 @base.NovaObjectRegistry.register
 class InstanceList(base.ObjectListBase, base.NovaObject):
     # Version 2.0: Initial Version
diff --git a/nova/tests/functional/db/test_instance.py b/nova/tests/functional/db/test_instance.py
index 75fcce8577d8..38b530134ac8 100644
--- a/nova/tests/functional/db/test_instance.py
+++ b/nova/tests/functional/db/test_instance.py
@@ -10,8 +10,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from oslo_utils import uuidutils
+
 from nova.compute import vm_states
 from nova import context
+from nova import db
 from nova import objects
 from nova import test
 
@@ -40,3 +43,48 @@ class InstanceObjectTestCase(test.TestCase):
             self.context, self.context.project_id, self.context.user_id,
             vm_states.ACTIVE)
         self.assertEqual(1, count)
+
+    def test_populate_missing_availability_zones(self):
+        # create two instances once with avz set and other not set.
+        inst1 = self._create_instance(host="fake-host1")
+        uuid1 = inst1.uuid
+        inst2 = self._create_instance(availability_zone="fake",
+                                      host="fake-host2")
+        self.assertIsNone(inst1.availability_zone)
+        self.assertEqual("fake", inst2.availability_zone)
+        count_all, count_hit = (objects.instance.
+            populate_missing_availability_zones(self.context, 10))
+        # we get only the instance whose avz was None.
+        self.assertEqual(1, count_all)
+        self.assertEqual(1, count_hit)
+        inst1 = objects.Instance.get_by_uuid(self.context, uuid1)
+        # since instance.host was None, avz is set to
+        # CONF.default_availability_zone i.e 'nova' which is the default zone
+        # for compute services.
+        self.assertEqual('nova', inst1.availability_zone)
+
+        # create an instance with avz as None on a host that has avz.
+        host = 'fake-host'
+        agg_meta = {'name': 'az_agg', 'uuid': uuidutils.generate_uuid(),
+                    'metadata': {'availability_zone': 'nova-test'}}
+        agg = objects.Aggregate(self.context, **agg_meta)
+        agg.create()
+        agg = objects.Aggregate.get_by_id(self.context, agg.id)
+        values = {
+            'binary': 'nova-compute',
+            'host': host,
+            'topic': 'compute',
+            'disabled': False,
+        }
+        service = db.service_create(self.context, values)
+        agg.add_host(service['host'])
+        inst3 = self._create_instance(host=host)
+        uuid3 = inst3.uuid
+        self.assertIsNone(inst3.availability_zone)
+        count_all, count_hit = (objects.instance.
+            populate_missing_availability_zones(self.context, 10))
+        # we get only the instance whose avz was None i.e inst3.
+        self.assertEqual(1, count_all)
+        self.assertEqual(1, count_hit)
+        inst3 = objects.Instance.get_by_uuid(self.context, uuid3)
+        self.assertEqual('nova-test', inst3.availability_zone)
diff --git a/releasenotes/notes/migration-tool-to-populate-inst.avz-29fed2fe57a9764d.yaml b/releasenotes/notes/migration-tool-to-populate-inst.avz-29fed2fe57a9764d.yaml
new file mode 100644
index 000000000000..409234e115ee
--- /dev/null
+++ b/releasenotes/notes/migration-tool-to-populate-inst.avz-29fed2fe57a9764d.yaml
@@ -0,0 +1,10 @@
+---
+upgrade:
+  - |
+    A new online data migration has been added to populate missing
+    instance.availability_zone values for instances older than Pike whose
+    availability_zone was not specified during boot time. This can be run
+    during the normal ``nova-manage db online_data_migrations`` routine.
+    This fixes `Bug 1768876`_
+
+    .. _Bug 1768876: https://bugs.launchpad.net/nova/+bug/1768876