Nakhon Si Thammarat province is a province (changwat) of Thailand, on the western shore of the Gulf of Thailand. As of 2018, it was the most populous province of Southern Thailand, with a population of approximately 1.5 million. Neighboring provinces are Songkhla, Phatthalung, Trang, Krabi and Surat Thani.

The provinces of Thailand are administrative divisions of the government of Thailand. The country is divided into 76 provinces proper, with one additional special administrative area. They are the primary local government units and act as juristic persons. They are divided into amphoe (districts) which are further divided into tambon, the next lower level of local government.