San Sai (Thai: สันทราย) is a district (amphoe) in the central part of Chiang Mai province in northern Thailand.

During the annual rainy season (monsoons), the Ping and the Khao River carry sediment that accumulates at the confluence of the two rivers, forming large dunes.

When the Siamese government of created this district on 20 October 1897, they named it San Sai.

Neighboring districts are (from the north clockwise) Mae Taeng, Doi Saket, San Kamphaeng, Mueang Chiang Mai and Mae Rim and of Chiang Mai Province.

The district is divided into 12 sub-districts (tambons), which are further subdivided into 116 villages (mubans).