Bay of Mali Ston (Croatian: Malostonski zaljev) is a bay in the Adriatic Sea, enclosed by the Pelješac peninsula and the mainland. The name primarily refers to the innermost, eastern part of the bay, while the name Channel of Mali Ston (Croatian: Malostonski kanal) is used for the channel between Klek (peninsula) and Pelješac, in the west. The two bodies of water are located in Croatia's and partly in Bosnia and Herzegovina's territorial waters.
The bay is named after the village of Mali Ston (near Ston). Its southernmost point is at the isthmus of the Pelješac peninsula, while its mouth is located roughly north of Hodilje. The Channel, in turn, extends from there to the line between Cape Blaca on Pelješac and Cape Međed on the shore, located south of Komarna. The sea to the west of the Mali Ston Channel, near the Neretva Delta, is called the "Malo more", and further west is the Neretva Channel.
It is 21 km (13 mi) long and has a maximum breadth of 2.2 km (1.4 mi) and the depth is between 7 and 28 meters. On account of its clean waters, the bay of Mali Ston was declared a Special Natural Reserve in 1983. An area of 4821 ha is covered, from the line Sreser-Duba eastwards. The Croatian Ministry of Agriculture considers this larger body of water as the total area of the bay for the purposes of regulating mariculture, with a length of 28 km (17 mi), a maximum breadth of 6.1 km (3.8 mi), and the depth largely between 20 and 29 meters.
The part of the bay that is in Bosnia and Herzegovina is a section in the northwest of the bay near Neum, and it also constitutes said country's only coastline. The maritime boundary between the two is, according to the common standards for territorial waters, an equidistant line between the Pelješac peninsula of Croatia in the southwest and the Klek peninsula in Bosnia and Herzegovina in the northeast.