diff --git a/google/cloud/bigquery/_helpers.py b/google/cloud/bigquery/_helpers.py
index ad8e3f003..4fe29291d 100644
--- a/google/cloud/bigquery/_helpers.py
+++ b/google/cloud/bigquery/_helpers.py
@@ -363,6 +363,11 @@ def _time_to_json(value):
     "DATETIME": _datetime_to_json,
     "DATE": _date_to_json,
     "TIME": _time_to_json,
+    # Make sure DECIMAL and BIGDECIMAL are handled, even though
+    # requests for them should be converted to NUMERIC.  Better safe
+    # than sorry.
+    "DECIMAL": _decimal_to_json,
+    "BIGDECIMAL": _decimal_to_json,
 }
 
 
diff --git a/google/cloud/bigquery/enums.py b/google/cloud/bigquery/enums.py
index b378f091b..787c2449d 100644
--- a/google/cloud/bigquery/enums.py
+++ b/google/cloud/bigquery/enums.py
@@ -203,8 +203,8 @@ class SqlTypeNames(str, enum.Enum):
     INT64 = "INTEGER"
     FLOAT = "FLOAT"
     FLOAT64 = "FLOAT"
-    NUMERIC = "NUMERIC"
-    BIGNUMERIC = "BIGNUMERIC"
+    DECIMAL = NUMERIC = "NUMERIC"
+    BIGDECIMAL = BIGNUMERIC = "BIGNUMERIC"
     BOOLEAN = "BOOLEAN"
     BOOL = "BOOLEAN"
     GEOGRAPHY = "GEOGRAPHY"  # NOTE: not available in legacy types
@@ -227,6 +227,8 @@ class SqlParameterScalarTypes:
     FLOAT64 = ScalarQueryParameterType("FLOAT64")
     NUMERIC = ScalarQueryParameterType("NUMERIC")
     BIGNUMERIC = ScalarQueryParameterType("BIGNUMERIC")
+    DECIMAL = ScalarQueryParameterType("NUMERIC")
+    BIGDECIMAL = ScalarQueryParameterType("BIGNUMERIC")
     BOOLEAN = ScalarQueryParameterType("BOOL")
     BOOL = ScalarQueryParameterType("BOOL")
     GEOGRAPHY = ScalarQueryParameterType("GEOGRAPHY")