Khun Si (, ) is one of the seven subdistricts (tambon) of Sai Noi District, in Nonthaburi Province, Thailand. Neighbouring subdistricts are (from north clockwise) Sai Yai, Rat Niyom, Khlong Khwang, Sai Noi, Thawi Watthana, Naraphirom and Bang Phasi. In 2020 it had a total population of 5,054 people.
The subdistrict is subdivided into 8 administrative villages (muban).
The whole area of the subdistrict is covered by Khun Si Subdistrict Administrative Organization ().