Phen (Thai: เพ็ญ, pronounced [pʰēn]) is a district (amphoe) in the northern part of Udon Thani province, Isan, Thailand.

Neighboring districts are (from the northeast clockwise) Sang Khom, Ban Dung, Phibun Rak, Mueang Udon Thani, Ban Phue of Udon Thani Province, Sakhrai, Mueang Nong Khai and Phon Phisai of Nong Khai province.

In 1907 it became a district, then named Mueang Phen (เมืองเพ็ญ) in Udon Thani Province.

There are a further 11 tambon administrative organizations (TAO).

This Udon Thani Province location article is a stub.