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 .«Река Айдар (Адар, Айдор)», Russian State Water Registry 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 / km². The river freezes in December.
The Aidar Battalion in the Ukrainian Armed Forces is named after this river.
References М. Т. Янко. Топонімічний словник-довідник української РСР, К., «Радянська школа», 1973, стор. 13