The Balkans are connected by water on three sides : the Black Sea to the east and branches of the Mediterranean Sea to the south and west ( including the Adriatic , Ionian , Aegean and Marmara seas ) . The Balkan Peninsula may be defined as an area of southeastern Europe surrounded by water on three sides : the Adriatic Sea to the west , the Mediterranean Sea ( including the Ionian and Aegean seas ) to the south and the Black Sea to the east .