Nakhon Si Thammarat is one of the southern provinces of Thailand, at the eastern shore of the Gulf of Thailand. Neighboring provinces are (from south clockwise) Songkhla, Phatthalung, Trang, Krabi and Surat Thani.
The province is located on the shore of the Gulf of Thailand on the east side of the Malay Peninsula. The terrain is mostly rugged hilly forest area. It contains the highest elevation of southern Thailand, the Khao Luang with 1,835 m, now protected in the Khao Luang national park.