The River Blackwater is a river in the English counties of Hampshire and Wiltshire. It is a tributary of the River Test.
The river rises just to the east of the Wiltshire village of Redlynch, near Salisbury. It then flows east across the county boundary into Hampshire. Here it flows north of the village of Wellow and Wigley before joining the Test between Totton and Redbridge.
This River Blackwater should not be confused with the River Blackwater in north-east Hampshire, which is a tributary of the River Loddon and (indirectly) of the River Thames.
Another similarly named waterway in Hampshire is Black Water. Black Water is a small stream which flows eastwards across the New Forest, passing under Rhinefield Ornamental Drive, before joining Ober Water and Highland Water just north of Brockenhurst to form the Lymington River.