Dan Sai (Thai: ด่านซ้าย; IPA: [dàːn sáːj]) is a district (amphoe) in the western part of Loei province, northeastern Thailand.

A large part of the district is a portion of the Phu Hin Rong Kla National Park.

Phu Thap Buek, the highest mountain of the Phetchabun Range, is at the southern end of the district.

The northern part of the district reaches the southern end of the Luang Prabang Range mountain area of the Thai highlands.

[2] According to General Apirat Kongsompong, on 5 November 1976, King Rama X, then an army captain and crown prince, engaged with communist insurgents in Ban Mak Kheng of Dan Sai District.