Sai Noi (, ) is one of the seven subdistricts (tambon) of Sai Noi District, in Nonthaburi Province, Thailand. Neighbouring subdistricts are (clockwise from north) Khlong Khwang, Lahan, Bang Bua Thong, Thawi Watthana and Khun Si. In 2020 it had a total population of 29,256 people.
The subdistrict is subdivided into 11 administrative villages (muban).
The area of the subdistrict is shared by two local administrative organizations.