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. This area is now the site of the San Sai District office. When the Siamese government of created this district on 20 October 1897, they named it San Sai. San is the Thai word for 'dune', and sai means 'sand'.