Bang Sai (Thai: บางไทร, pronounced [bāːŋ sāj]) is a district (amphoe) in the southern 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.

The district dates back to khwaeng Sena Noi, which became an amphoe in 1898.

The topography of Bang Sai is a lowland area, so it experiences flooding almost every year.

Bang Sai has a population of about 50,000 people, most of whom are engaged in agriculture, and few of them work in the local industrial factories.