Bang Sai (Thai: บางซ้าย, pronounced [bāːŋ sáːj]) is a district (amphoe) in the western part of Phra Nakhon Si Ayutthaya province, central Thailand.

There is another district of Ayutthaya which shares the same romanization Bang Sai, but has a different spelling in Thai.

[2] Neighboring districts are (from the north clockwise) Phak Hai, Sena and Lat Bua Luang of Ayutthaya Province, and Bang Pla Ma of Suphanburi province.

The district is divided into six sub-districts (tambons), which are further subdivided into 53 villages (mubans).

The non-municipal area is administered by four tambon administrative organizations (TAO).