The Tundzha (Bulgarian: Тунджа [ˈtund͡ʒɐ]; Turkish: Tunca [tund͡ʒa]; Greek: Τόνζος [ˈtonzos]) 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, initially flowing east and then making a sharp turn to the south before Yambol, in which direction it flows until it reaches the Maritsa.

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.

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