Dorchester Castle was in the market town of Dorchester, Dorset, southern England (). This motte and bailey castle stood in the north part of the town. Between 1154 and 1175 it was in possession of the Earl of Cornwall and it had become a royal possession by 1185. It appears to have been disused from about 1290 and there are references to its stonework being reused to build Dorchester Greyfriars in 1309. There are now no remains and Dorchester Prison occupies its site. Great Western Railway Castle-class locomotive No. 4090 was named after the castle.