Phak Hai (Thai: ผักไห่, pronounced [pʰàk hàj]) is a district (amphoe) in the northwestern part of Ayutthaya province, central Thailand.

Historically, the district was named Khwaeng Sena Yai, which became converted to an amphoe at the end of the 19th century.

In 1917, the name of district was changed to be Phak Hai after the central tambon.

The spelling has changed however now, so it can also mean a kind of plant (Momordica charantia) that grows in swamps.

The district is divided into 16 sub-districts (tambon), which are further subdivided into 128 villages (muban).