The River Blackwater is a tributary of the Loddon in England and sub-tributary of the Thames. It rises at two springs in Rowhill Nature Reserve between Aldershot, Hampshire and Farnham, Surrey. It curves a course north then west to join the Loddon in Swallowfield civil parish, central Berkshire. Part of the river splits Hampshire from Surrey; a smaller part does so as to Hampshire and Berkshire.

Blackwater is a small town in the northeastern corner of Hampshire, England, lying in the county's Hart District. Considered to be part of the Farnborough/Aldershot Built-up Area and almost contiguous with Camberley, Surrey, it is centred 32 miles (51 km) WSW from London.