| Official Name: | Bang Sai |
| Native Name: | บางซ้าย |
| Native Name Lang: | th |
| Settlement Type: | District |
| Subdivision Type: | Country |
| Subdivision Name: | Thailand |
| Subdivision Type1: | Province |
| Subdivision Name1: | Ayutthaya |
| Subdivision Type2: | Seat |
| Subdivision Name2: | Bang Sai |
| Subdivision Type3: | Tambon |
| Subdivision Type4: | Muban |
| Established Title: | District established |
| Population Total: | 19579 |
| Population As Of: | 2007 |
| Population Density Km2: | 129.9 |
| Area Total Km2: | 150.756 |
| Blank Name Sec1: | Postal code |
| Blank Info Sec1: | 13270 |
| Blank Name Sec2: | Geocode |
| Blank Info Sec2: | 1413 |
| Timezone: | ICT |
| Utc Offset: | +7 |
| Coordinates: | 14.32°N 100.3°W |
Bang Sai (Thai: บางซ้าย, in Thai pronounced as /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.
The district was created as a minor district (king amphoe) in 1948 with territory taken from Sena district.[1] It was upgraded to a full district in 1958.[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 sub-district municipality (thesaban tambon) Bang Sai covers parts of the tambons Bang Sai, Kaeo Fa, and Tao Lao. The non-municipal area is administered by four tambon administrative organizations (TAO).
| No. | Name | Thai | Pop. | |
|---|---|---|---|---|
| 1. | Bang Sai | บางซ้าย | 3,389 | |
| 2. | Kaeo Fa | แก้วฟ้า | 2,429 | |
| 3. | Tao Lao | เต่าเล่า | 2,821 | |
| 4. | Plai Klat | ปลายกลัด | 4,774 | |
| 5. | Thepphamongkhon | เทพมงคล | 3,861 | |
| 6. | Wang Phatthana | วังพัฒนา | 2,305 |