The Aidar (Russian and ) is a river in Luhansk Oblast, Ukraine and Belgorod Oblast, Russia. A left tributary of the Seversky Donets, it is long (with located within Ukraine) and has a basin area of . The slopes of the valley are partitioned with ravines and gorges. In the spring, snow melt accounts for approximately 70% of the flow. The average flow module is 1.7 litres / sec / kmò. The river freezes in December.
The Aidar Battalion in the Ukrainian Armed Forces is named after this river.