Borzya (Russian: Борзя) is a river in Zabaykalsky Krai, Russia. It is a right tributary of the Onon River (in Amur's basin). It is 304 km long, with a drainage basin of 7080 km².
The Borzya has its sources in the Kukulbey Mountains. It flows through a wide, swampy valley in a steppe landscape. The river's waters comes mainly from rain, and there are periodic flooding in the summer. The river sometimes dries up completely. The average discharge is 2,8 m³/sek.
The town of Borzya lies along the river.