The Gashnya (Bulgarian: Гашня) is a 20 km long river in southern Bulgaria, flowing through Pazardzhik and Smolyan Provinces.

The river springs at an altitude of 2,018 m from the southeastern slopes of the summit of Batashki Snezhnik (2,082 m) in the Batak Mountain of the western Rhodope Mountains.

At an altitude of 689 m the Gashnya flows into the Tsankov Kamak Reservoir, just upstream of its dam, constructed on the river Vacha.

[1] Its drainage basin covers a territory of 54 km2,[1] or 3.3% of the Vacha's total.

Part of the waters along its upper course are diverted west to be utilized for electricity generation at the Batak Hydropower Cascade (254 MW), while the rest are utilized at the Dospat–Vacha Hydropower Cascade (500.2 MW).