Mae Sai (Thai: แม่สาย, pronounced [mɛ̂ː sǎːj]; Shan: မႄႈသၢႆ, pronounced [mɛ.sʰǎj]), is the district town of Mae Sai District in the far north of Chiang Rai Province, Thailand. Mae Sai is a major border crossing between Thailand and Myanmar; the town of Tachileik, in Shan State is across the bridge. Asian Highway Network AH2 (Thailand Route 1 or Phahonyothin Road) crosses the Mae Sai River to the town of Tachileik in Myanmar . The town of Mae Sai and Tachileik are the bi-national conurbation shared between Thailand and Myanmar.

The Mae Sai sanitation district was created on 14 May 1956 It was upgraded to a municipality on 25 May 1999. The municipality governs neighbourhoods (mu) 1, 2, 3, 10 Wiang Phang Kham subdistrict and Mu 2, 6, 7, 8, 10 Mae Sai Subdistrict

Neighboring districts are (from the north clockwise): Mae Sai River and Myanmar border; Mae Sai Mittrapab Subdistrict Municipality; Wiang Phang Kham Subdistrict Municipality.

The Tham Luang Nang Non cave is within the subdistrict.