The Rezovo (also Rezovska, Rezvaya and Rezve; Bulgarian: Резовска река [ˈrɛzofskɐ rɛˈka]; Turkish: Mutludere [mutˈludeɾe]) is a river in the extreme southeast of Bulgaria and northernmost part of European Turkey. The river is situated in Bulgaria's Strandzha Nature Park.

The Rezovo is 112 km long and its source is in the Turkish part of the Strandzha mountains east of Kofçaz in Turkey under the Turkish name of Paspalderesi. It generally flows east from Turkey to Bulgaria, and after Paspala village of Kırklareli Province in Turkey becomes a border river between the Turkey and Bulgaria until its mouth in the Black Sea at Rezovo. This location 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's catchment spreads over 739 km, of which 555 km are in Turkey, and 184 km in Bulgaria.

The border at the Rezovo'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.