Khai Si (Thai: ไคสี) is a sub-district (tambon) in Mueang Bueng Kan District, in Bueng Kan Province, northeastern Thailand. As of 2012, it had a population of 5,294 people. It lies on Highway 212 and the Mekong River and border with Laos, west of Bueng Kan and east of Ho Kham.