Kosamphi Nakhon (Thai: โกสัมพีนคร, pronounced [kōː.sǎm.pʰīː ná(ʔ).kʰɔ̄ːn]) is a district (amphoe) in the northwestern part of Kamphaeng Phet province, central Thailand.

Archaeological evidence of the ancient city was found in Ban Khlong Mueang, Tambon Kosamphi.

[3] Neighboring districts are (from the northeast clockwise) Phran Kratai, Mueang Kamphaeng Phet of Kamphaeng Phet Province and Wang Chao and Mueang Tak of Tak province.

Important water resources are the Ping and Wang Chao River.

The district is divided into three sub-districts (tambons), which are further subdivided into 43 villages (mubans).