diff --git a/oslo_serialization/jsonutils.py b/oslo_serialization/jsonutils.py
index a2a700b..77598b2 100644
--- a/oslo_serialization/jsonutils.py
+++ b/oslo_serialization/jsonutils.py
@@ -37,7 +37,6 @@ import io
 import itertools
 import json
 import uuid
-import warnings
 from xmlrpc import client as xmlrpclib
 
 from oslo_utils import encodeutils
@@ -173,13 +172,10 @@ def to_primitive(value, convert_instances=False, convert_datetime=True,
         # __iter__ defined but it isn't callable as list().
         return fallback(value)
 
-    if orig_fallback is not None:
-        return orig_fallback(value)
+    if orig_fallback is None:
+        raise ValueError("Cannot convert %r to primitive" % (value,))
 
-    # TODO(gcb) raise ValueError in version 3.0
-    warnings.warn("Cannot convert %r to primitive, will raise ValueError "
-                  "instead of warning in version 3.0" % (value,))
-    return value
+    return orig_fallback(value)
 
 
 JSONEncoder = json.JSONEncoder
diff --git a/oslo_serialization/tests/test_jsonutils.py b/oslo_serialization/tests/test_jsonutils.py
index 5599d91..74289b9 100644
--- a/oslo_serialization/tests/test_jsonutils.py
+++ b/oslo_serialization/tests/test_jsonutils.py
@@ -261,8 +261,7 @@ class ToPrimitiveTestCase(test_base.BaseTestCase):
         p = jsonutils.to_primitive(x)
         self.assertEqual({'a': 1, 'b': 2, 'c': 3}, p)
 
-    @mock.patch('warnings.warn')
-    def test_instance(self, warn_mock):
+    def test_instance(self):
         class MysteryClass(object):
             a = 10
 
@@ -273,8 +272,7 @@ class ToPrimitiveTestCase(test_base.BaseTestCase):
         self.assertEqual(dict(b=1),
                          jsonutils.to_primitive(x, convert_instances=True))
 
-        self.assertEqual(x, jsonutils.to_primitive(x))
-        warn_mock.assert_called_once()
+        self.assertRaises(ValueError, jsonutils.to_primitive, x)
 
     def test_typeerror(self):
         x = bytearray  # Class, not instance
@@ -354,8 +352,7 @@ class ToPrimitiveTestCase(test_base.BaseTestCase):
     def test_fallback(self):
         obj = ReprObject()
 
-        ret = jsonutils.to_primitive(obj)
-        self.assertIs(obj, ret)
+        self.assertRaises(ValueError, jsonutils.to_primitive, obj)
 
         ret = jsonutils.to_primitive(obj, fallback=repr)
         self.assertEqual('repr', ret)
@@ -364,8 +361,7 @@ class ToPrimitiveTestCase(test_base.BaseTestCase):
         obj = ReprObject()
         obj_list = [obj]
 
-        ret = jsonutils.to_primitive(obj_list)
-        self.assertEqual([obj], ret)
+        self.assertRaises(ValueError, jsonutils.to_primitive, obj_list)
 
         ret = jsonutils.to_primitive(obj_list, fallback=repr)
         self.assertEqual(['repr'], ret)
diff --git a/releasenotes/notes/jsonutils-to_primitive-value-error-89338f90310e9518.yaml b/releasenotes/notes/jsonutils-to_primitive-value-error-89338f90310e9518.yaml
new file mode 100644
index 0000000..4329ed5
--- /dev/null
+++ b/releasenotes/notes/jsonutils-to_primitive-value-error-89338f90310e9518.yaml
@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    The ``oslo_utils.jsonutils.to_primitive`` function now raises ValueError
+    when the input value can't be converted.