Lak Si (Thai: หลักสี่, pronounced [làk sìː]) is one of the 50 districts (khet) of Bangkok, Thailand.

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.

Lak Si station, an interchange station on SRT Dark Red Line and MRT Pink Line, serves the district.

The community here was called Lak Si because it is at the fourth milestone of Khlong Prem Prachakon (คลองเปรมประชากร) (canal) dug during King Chulalongkorn's reign linking Bangkok to Bang Pa-in.

This article related to the city of Bangkok, Thailand is a stub.