Nakhon Si Thammarat is one of the southern provinces (changwat) 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 name of the province derives from its Pali-Sanskrit name Nagara Sri Dhammaraja (City of the Sacred Dharma King), or Bandar Sri Raja Dharma in Malay, which in Thai pronunciation becomes Nakhon Si Thammarat.