Sai Noi (Thai: ไทรน้อย, pronounced [sāj nɔ́ːj]) is the northwesternmost district (amphoe) of Nonthaburi province, central Thailand.

Neighboring districts are (from the north clockwise): Lat Bua Luang (Phra Nakhon Si Ayutthaya province); Lat Lum Kaeo (Pathum Thani province); Bang Bua Thong, Bang Yai, and the districts Phutthamonthon and Bang Len of Nakhon Pathom province.

On 1 January 1948 four sub-districts were split off to form the new minor district (king amphoe) Sai Noi.

[2] In 1959 Tambon Khun Si was transferred from Bang Len District (Nakhon Pathom) to become part of Sai Noi.

The district is divided into seven sub-districts (tambons), which are further subdivided into 68 villages (mubans).