Lake Mandrensko or Lake Mandra (Bulgarian: Мандренско езеро, Mandrensko ezero) is the southernmost of the Burgas Lakes, located in the immediate proximity of the Black Sea and close to Burgas. Being 8 km long and 1.3 km wide at maximum, as well as having an area of 38.84 km², it was a brackish natural lake until 1963, when it was turned into a reservoir with the construction of a dam to secure fresh water for the large Neftochim Burgas oil refinery.