Phon Phisai (, ; , ) is a district (amphoe) in Nong Khai province, northeastern Thailand.
The district was originally part of Mueang Phon Phaeng, now part of Rattanawapi district. In 1906 it was established as a district.
Neighboring districts are (from the east clockwise) Rattanawapi and Fao Rai of Nong Khai Province; Ban Dung, Sang Khom, and Phen of Udon Thani province; and Mueang Nong Khai of Nong Khai. To the north across the Mekong River are the Laotian Vientiane Prefecture and Bolikhamxai province.
The district is divided into 11 sub-districts (tambons), which are further subdivided into 151 villages (mubans). Chumphon is a sub-district municipality (thesaban tambon) which covers parts of tambon Chumphon. There are a further 11 tambon administrative organizations (TAO).
<small>Missing numbers are tambon which now form the districts Fao Rai and Rattanawapi.</small>