The Rezovo River (also Rezovska River, Rezvaya and Rezve; Bulgarian: Резовска река, Rezovska reka; Turkish: Mutludere) is a river in southeasternmost Bulgaria and northernmost (European) Turkey. The river is situated in Bulgaria's Strandzha Nature Park.
112 km in length, it takes its source from the Turkish part of the Strandzha mountains east of Kofçaz under the name of Paspalderesi. It generally flows east and after Paspala becomes a border river between the two countries until its mouth in the Black Sea at Rezovo, which constitutes the southernmost point of the Bulgarian Black Sea Coast and the northernmost point of the Turkish one, as well as the southeasternmost point of Bulgaria and northeasternmost point of Turkish Thrace. The Rezovo River's whatershed spreads over 738 km², of which 555 in Turkey. Its largest tributary is the Velika.
The border at the Rezovo River's mouth was the subject of a minor territorial dispute between Bulgaria and Turkey, which was settled in the 1990s. As a result of an agreement between the two countries of 6 May 1992 (ratified by Bulgaria in 1998), Bulgaria received a small land area of several square kilometres in the Rezovo Bay in return for water area in the continental shelf.
Rezovo (Bulgarian: Резово, pronounced [ˈrɛzuvu]) is a village and seaside resort in southeastern Bulgaria, part of Tsarevo Municipality, Burgas Province, in the coastal Strandzha geographical region. The village is situated in Strandzha Nature Park. Lying at the mouth of the Rezovo River in the Black Sea, Rezovo is the southernmost point of the Bulgarian Black Sea Coast and the southeastern-most inhabited place in Bulgaria and the European Union mainland. The village is located 11 kilometres south of Sinemorets, 17 km from Ahtopol and 36 km from Lozenets. As the Rezovo River constitutes the border between Bulgaria and the European part of Turkey, Rezovo directly overlooks the Turkish bank of the river and the Turkish village of Beğendik, in Demirköy district, Kırklareli Province.
Researchers have attempted to link the village's name to that of Rhesus of Thrace, a Thracian king of the Iliad, and although the existence of an ancient settlement at the place of the modern village has been proven (with the oldest artifacts dating to the 4th century BC), it is hard to establish any connection with Rhesus' personality. In ancient times, the area was subject to large-scale metallurgical activity, with the largest ancient deposit of slag in Bulgaria. It is believed that the medieval fortress of Castrition lay on Cape Kastrich north of modern Rezovo.