The Bolgasyn (Kazakh: Болғасын) is a river in Shalkar District, Aktobe Region, Kazakhstan.

[1] The Bolgasyn has its sources in the Shagyray Plateau, part of the southern sector of the Mugalzhar Hills.

It heads first southeastwards, parallel to the upper course of the Kauylzhyr river that flows a little to the south, to the northwest of lake Shalkar.

[2][3][1] The Bolgasyn usually reaches its highest level for a few weeks in March and April when the snows melt.

Its floodplain has 38 small tributaries with a total length of 10 km (6.2 mi).