Tundzha (in ancient times Tonzos, Tonsus) is a river in southern Bulgaria, districts of Plovdiv, Stara Zagora, Sliven, Yambol and Haskovo and Turkey, and Edirne vilayet.

Its length is 390 km, of which on Bulgarian territory - 349,5 km, which ranks 3rd in the rivers of Bulgaria after the Danube and Iskar. The Tundzha River is Maritsa's largest tributary.

The Tundzha River starts at 2083 m above sea level. under the name of Selska Reka, 250 m south of the Yurushka Gramada Peak (2136 m) in the Kalofer Mountain of Stara Planina.

The town of Kalofer flows southwards into a deeply cut valley with steep slopes and a long slope. Its transverse profile has a width at the bottom of the valley of about 50 - 60 m. The channel is a stony-pebble.

The speed of the water flow is large - 1.5 - 2 m / s.