Nakhon Ratchasima (Thai: นครราชสีมา, pronounced [náʔkʰɔːn râːt.tɕʰa.sǐː.maː]) or (Khmer: Nokor Reach Seyma) is a city (thesaban nakhon) in the northeast (Isan) of Thailand and gateway to Isan. It is the capital of the Nakhon Ratchasima Province and Nakhon Ratchasima district. As of August 16, 2010, the municipal area has a population of 142,645.

The city is also commonly known as Korat (RTGS Khorat, โคราช), a short version of the full name. It is located at the western edge of the Khorat Plateau and historically marked the boundary between the Lao and Siamese territory, however now is considered a gateway to the Northeast (Isan). Its geographical location is 14°58.5′N 102°6′E / 14.975°N 102.1°E / 14.975; 102.1Coordinates: 14°58.5′N 102°6′E / 14.975°N 102.1°E / 14.975; 102.1.

Nakhon Ratchasima is also the center of the Nakhon Ratchasima Metropolitan Area.

Archeological evidence suggests that there were two ancient towns that later became named Sema and Nakhon Raj which currently are in Sung Noen district, 32 km west of present-day Nakhonratchasima (Korat). Nakhon means capital, Raj means kingdom or state in Hindi languages. See also Bai Sema.