Koster is a small farming town situated on the watershed between the Limpopo and Orange Rivers in the North West Province of South Africa.
The town lies about 58 km south-west of Rustenburg and 72 km west-north-west of Magaliesburg.
Koster is part of Kgetlengrivier Local Municipality under the Bojanala Platinum District Municipality of North West Province.
Koster was founded on the farm Kleinfontein in 1913, and has been administered by a village council since January 1931. It is said to have been named either after Dr Herman Jacob Coster (1866–1899), State Attorney of the South African Republic, after its surveyor, or after Bastiaan Hendricus Koster, owner of the farm Kleinfontein. The latter explanation seems most plausible.
Koster means “church sexton”.
The Elands River, the Koster River and the Mooi River have their sources near the town.