Lak Si is one of the 50 districts (khet) of Bangkok. It is bounded by (from the north clockwise) Don Mueang, Bang Khen and Chatuchak of Bangkok; Mueang Nonthaburi District and Pak Kret District of Nonthaburi Province.
The name Lak Si means 'fourth milestone'. The community here was called Lak Si because it is at the fourth milestone of Khlong Prem Prachakon (canal) dug during King Chulalongkorn's reign linking Bangkok to Bang Pa-in. Lak Si became a district on 21 November 1997, separating from Don Mueang District. The district is divided into two sub-districts (khwaeng), Thung Song Hong and Talat Bang Khen.