Mueang Nakhon Si Thammarat (, ) is the capital district (amphoe mueang) of Nakhon Si Thammarat province in southern Thailand.
Neighboring districts are (from the southeast clockwise): Pak Phanang, Chaloem Phra Kiat, Phra Phrom, Lan Saka, Phrom Khiri and Tha Sala. To the east is the Gulf of Thailand.
The district is divided into 16 sub-districts (tambons), which are further subdivided into 114 villages (mubans).
<small>The missing numbers 9-11 and 17 belong to communes which were split off in 1994 to form Phra Phrom District.</small>
Nakhon Si Thammarat itself has city status (thesaban nakhon) and covers tambons Nai Mueang, Tha Wang, Khlang, and parts of Na Khian and Pho Sadet. There are a further three townships (thesaban tambons): Bang Chak covering parts of the same-named tambon, Tha Phae covering parts of tambon Pak Phun, and Pak Nakhon covering parts of tambons Tha Rai and Pak Nakhon. There are also 13 tambon administrative organizations (TAO).