| Official Name: | Lak Si |
| Native Name: | หลักสี่ |
| Native Name Lang: | th |
| Settlement Type: | Khet |
| Coordinates: | 13.8875°N 100.5789°W |
| Subdivision Type: | Country |
| Subdivision Name: | Thailand |
| Subdivision Type1: | Province |
| Subdivision Name1: | Bangkok |
| Subdivision Type2: | Seat |
| Subdivision Name2: | Thung Song Hong |
| Subdivision Type3: | Khwaeng |
| Subdivision Name3: | 2 |
| Established Title: | Khet established |
| Established Date: | 21 November 1997 |
| Area Total Km2: | 22.841 |
| Population Total: | 104,701[1] |
| Population As Of: | 2017 |
| Population Density Km2: | 4,583.90 |
| Timezone: | ICT |
| Utc Offset: | +7 |
| Postal Code Type: | Postal code |
| Postal Code: | 10210 |
| Geocode: | 1041 |
Lak Si (Thai: หลักสี่, in Thai pronounced as /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.
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 (คลองเปรมประชากร) (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).
| No. | Name | Thai | Area (km2) | Map |
|---|---|---|---|---|
| Thung Song Hong | Thai: ทุ่งสองห้อง | 16.886 | ||
| Talat Bang Khen | Thai: ตลาดบางเขน | 5.955 | ||
| Total | 22.841 | |||