Casterton is a town in Victoria, Australia, located on the Glenelg Highway, 42 kilometres east of the South Australian border, in the Shire of Glenelg. The Glenelg River passes through the town. Casterton is named after the village of Casterton in south-east Cumbria in England.

The Shire of Glenelg is a local government area in the Barwon South West region of Victoria, Australia, located in the south-western part of the state. It covers an area of 6,219 square kilometres (2,401 sq mi) and in June 2018 had a population of 19,665. It includes the towns of Casterton, Heywood, Merino and Portland. Although a shire of the same name existed before the amalgamations of the mid-1990s, the current Shire was formed in 1994 from the amalgamation of the former Shire of Glenelg with the Shire of Heywood and City of Portland.