Bang Si Thong (, ) is one of the nine subdistricts (tambon) of Bang Kruai District, in Nonthaburi Province, Thailand. The subdistrict is bounded by (clockwise from north) Bang Krang, Bang Si Mueang, Bang Phai, Bang Kruai, Wat Chalo and Bang Khanun subdistricts. In 2020, it had a total population of 11,426 people.
The subdistrict is subdivided into 5 administrative villages (muban).
The whole area of the subdistrict is covered by Bang Si Thong Subdistrict Municipality ().