Lan Sak (Thai: ลานสัก, pronounced [lāːn sàk]) is a district (amphoe) in northern Uthai Thani province, northern Thailand.

Tambons Lan Sak and Pradu Yuen of Ban Rai district were combined as a minor district (king amphoe) on 15 October 1975. The minor district office was in Ban Pak Muean. It was upgraded to a full district on 13 July 1981. At the same time, the office was moved to Ban Kao.

Neighboring districts are (from the east clockwise) Sawang Arom, Thap Than, Nong Chang, Huai Khot, and Ban Rai of Uthai Thani Province; Mae Poen and Chum Ta Bong of Nakhon Sawan province.

The important water resources are the Huai Thap Salao reservoir and Kha Khaeng River (Huai Kha Khaeng).

The district is divided into six sub-districts (tambons), which are further subdivided into 81 villages (mubans). The township (thesaban tambon) Lan Sak covers parts of tambons Lan Sak and Pradu Yuen. There are a further six tambon administrative organizations (TAO).

No.	Name	Thai name	Villages	Pop.
1.	Lan Sak	ลานสัก	10	13,905
2.	Pradu Yuen	ประดู่ยืน	12	6,697
3.	Pa O	ป่าอ้อ	10	7,358
4.	Rabam	ระบำ	19	12,573
5.	Nam Rop	น้ำรอบ	18	9,635
6.	Thung Na Ngam	ทุ่งนางาม	12	7,380

Hup Pa Tat is a broad valley located in the north of Uthai Thani Province.

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