Sai Mun (Thai: ทรายมูล, pronounced [sāːj mūːn]) is a district of Yasothon province in northeastern Thailand.

[1] After the separation of Yasothon from Ubon in 1972, on 15 May 1975, Sai Mun was elevated to a minor district (king amphoe), taking four tambons from Mueang Yasothon District: Sai Mun, Du Lat, Dong Mafai, and Na Wiang.

[2] In 1978, tambon Phai was reassigned from Kut Chum district to King Amphoe Sai Mun.

Neighboring districts are (from the northeast clockwise) Kut Chum and Mueang Yasothon of Yasothon Province, and Selaphum of Roi Et province.

The district is divided into five sub-districts (tambons) with 53 villages (mubans).