diff --git a/lower-constraints.txt b/lower-constraints.txt
index 05a8d414..a280a450 100644
--- a/lower-constraints.txt
+++ b/lower-constraints.txt
@@ -48,6 +48,7 @@ requests==2.14.2
 requests-mock==1.2.0
 requestsexceptions==1.2.0
 rfc3986==0.3.1
+simplejson==3.13.2
 six==1.10.0
 smmap==0.9.0
 stestr==1.0.0
@@ -57,6 +58,5 @@ testrepository==0.0.18
 testscenarios==0.4
 testtools==2.2.0
 traceback2==1.4.0
-ujson==1.35
 unittest2==1.1.0
 wrapt==1.7.0
diff --git a/monasca_common/rest/utils.py b/monasca_common/rest/utils.py
index 13386efc..eebaa4f7 100644
--- a/monasca_common/rest/utils.py
+++ b/monasca_common/rest/utils.py
@@ -13,7 +13,7 @@
 # under the License.
 
 import six
-import ujson as json
+import simplejson as json
 
 from monasca_common.rest import exceptions
 
diff --git a/monasca_common/tests/validation/test_metric_validation.py b/monasca_common/tests/validation/test_metric_validation.py
index e5bfbd61..b0ef7808 100644
--- a/monasca_common/tests/validation/test_metric_validation.py
+++ b/monasca_common/tests/validation/test_metric_validation.py
@@ -410,7 +410,7 @@ class TestMetricValidation(base.BaseTestCase):
             metric_validator.InvalidValueMeta,
             "value combinations must be",
             metric_validator.validate, metric)
-        # TODO: what would make ujson.dumps fail?
+        # TODO: what would make json.dumps fail (simplejson)?
 
     def test_invalid_timestamp(self):
         metric = {'name': 'test_metric_name',
diff --git a/monasca_common/validation/metrics.py b/monasca_common/validation/metrics.py
index e75bc9b4..cc7b1be3 100644
--- a/monasca_common/validation/metrics.py
+++ b/monasca_common/validation/metrics.py
@@ -18,7 +18,7 @@ import re
 
 import six
 import sys
-import ujson
+import simplejson as json
 
 # This is used to ensure that metrics with a timestamp older than
 # RECENT_POINT_THRESHOLD_DEFAULT seconds (or the value passed in to
@@ -108,7 +108,7 @@ def validate_value_meta(value_meta):
             raise InvalidValueMeta(msg)
 
     try:
-        value_meta_json = ujson.dumps(value_meta)
+        value_meta_json = json.dumps(value_meta)
     except Exception:
         raise InvalidValueMeta("Unable to serialize valueMeta into JSON")
     if len(value_meta_json) > VALUE_META_VALUE_MAX_LENGTH:
diff --git a/requirements.txt b/requirements.txt
index 6808390e..f36b75f0 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -8,5 +8,5 @@ oslo.config>=5.2.0 # Apache-2.0
 oslo.policy>=1.30.0 # Apache-2.0
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 pyparsing>=2.1.0 # MIT
-ujson>=1.35 # BSD
+simplejson>=3.13.2 # MIT
 confluent-kafka>=0.11.4 # Apache-2.0