Lake Siutghiol runs parallel to Mamaia's beach, separated from the Black Sea by the narrow strip of land on which the resort sits. The lake is about 10 kilometres long and largely shallow, with reed beds along the southern and western shores. It is calmer than the sea, protected from wind, and popular with windsurfers, kayakers, and pedal-boat riders.
Boat trips on the lake are available from the promenade and offer a different perspective on the resort — and views toward Ovidiu Island in the lake's centre, which holds a small monastery. The lake also has therapeutic properties: its mud has been used for treatment at the nearby spa resort of Eforie Nord since the 19th century.