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thumb|right|The river in Horncastle
The River Bain is a river in Lincolnshire, England, and a tributary of the River Witham.
The Bain rises in the Lincolnshire Wolds at Ludford, a village on The Viking Way long-distance footpath, and flows through or past the villages of Burgh on Bain, Biscathorpe, Donington on Bain, Goulceby, Asterby and Hemingby before reaching the town of Horncastle where it is joined by the River Waring, which rises at Belchford, to the north east of Horncastle. roach and rudd, as well as brown trout, pike, eel, and smaller species such as miller's thumb (Cottus gobio), gudgeon and stone loach (Nœmacheilus barbatus). It is also home to the threatened species of crayfish Austropotamobius pallipes, though there are also populations of the introduced American signal crayfish (Pasifastacus leniusculus), which competes with the native species for food.
The Bain valley was formed by a glacier in the most recent ice-age and, although small, is very obvious. The river once again broke its banks in Horncastle during Storm Babet 20th October 2023.
Settlements in the valley include Tattershall, Coningsby, Kirkby on Bain, Haltham, Roughton, Horncastle, Hemingby, Goulceby with Asterby, Donington on Bain, Burgh on Bain and Ludford.