diff --git a/google/cloud/bigquery/_helpers.py b/google/cloud/bigquery/_helpers.py
index 2310224ae..47851d42c 100644
--- a/google/cloud/bigquery/_helpers.py
+++ b/google/cloud/bigquery/_helpers.py
@@ -19,6 +19,7 @@
 import datetime
 import decimal
 import re
+import six
 
 from google.cloud._helpers import UTC
 from google.cloud._helpers import _date_from_iso8601_date
@@ -447,15 +448,16 @@ def _record_field_to_json(fields, row_value):
         if isdict:
             processed_fields.add(subname)
 
-    # Unknown fields should not be silently dropped.
+    # Unknown fields should not be silently dropped, include them. Since there
+    # is no schema information available for them, include them as strings
+    # to make them JSON-serializable.
     if isdict:
         not_processed = set(row_value.keys()) - processed_fields
-        if not_processed:
-            raise ValueError(
-                "Unknown field(s) not present in schema: {}".format(
-                    ", ".join(not_processed)
-                )
-            )
+
+        for field_name in not_processed:
+            value = row_value[field_name]
+            if value is not None:
+                record[field_name] = six.text_type(value)
 
     return record
 
diff --git a/tests/unit/test__helpers.py b/tests/unit/test__helpers.py
index 4dd4a8520..c29bedd7b 100644
--- a/tests/unit/test__helpers.py
+++ b/tests/unit/test__helpers.py
@@ -916,10 +916,14 @@ def test_w_dict_unknown_fields(self):
             _make_field("INT64", name="one", mode="NULLABLE"),
             _make_field("STRING", name="two", mode="NULLABLE"),
         ]
-        original = {"whoami": "_?_?_", "one": 111, "two": "222"}
+        original = {"whoami": datetime.date(2020, 7, 20), "one": 111, "two": "222"}
 
-        with six.assertRaisesRegex(self, ValueError, r".*[Uu]nknown field.*whoami.*"):
-            self._call_fut(fields, original)
+        converted = self._call_fut(fields, original)
+
+        # Unknown fields should be included, but converted as strings.
+        self.assertEqual(
+            converted, {"whoami": "2020-07-20", "one": "111", "two": "222"},
+        )
 
 
 class Test_field_to_json(unittest.TestCase):