The Tundzha (Turkish: Tunca) is a river in Bulgaria and Turkey (known in antiquity as the Tonsus) and the most significant tributary of the Maritsa, emptying into it on Turkish territory near Edirne.
The river originates in the central parts of Stara Planina north of Kalofer, it then flows east and makes a sharp turn to the south before Yambol, in which direction it flows until it reaches the Maritsa.
The length is about 365 km, of which 328 km is on Bulgarian territory (including border). It has about 50 tributaries, the more important of which are Mochuritsa, Popovska and Sinapovska.
Towns on the banks of the river include Kalofer, Yambol and Elhovo.
