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[3] It was upgraded to a municipality on 25 May 1999.