The province is in the valley of the Mun River, a tributary of the Mekong. The Dângrêk mountain chain, which forms the border with Cambodia, is in the south of the province.
Khao Phra Wihan National Park covers an area of 130 km² of the Dângrêk mountains in the southeast of the province. Established on March 20, 1998, it is named after a ruined Khmer Empire temple Prasat Preah Vihear (anglicised in Thailand as Prasat Khao Phra Wihan), now in Cambodia, which had been the issue of boundary dispute. The temple faces north and was built to serve the Sisaket region. Earlier maps had all shown it as inside of Thailand. However, a boundary survey conducted by the French for the Franco-Siamese Treaty of 1907 deviated from the agreed-upon international divide by watershed in order to place the temple on the French (Cambodian) side.