Si Thoi, Phayao (Thai: ศรีถ้อย, pronounced [sǐː tʰɔ̂j]) is a tambon (subdistrict) of Mae Chai District, in Phayao Province, Thailand.

In 2014 it had a population of 7,038 people.

[1] The tambon is divided into 13 administrative villages (mubans).

The area of the subdistrict is shared by two local governments.

This Phayao Province location article is a stub.