Bolshoye Morskoye or Mainychin Ankavatyn (Russian: Большое Морское озеро; Chukot: Майнычин-Анкаватын, Maynıçın-Ankavatın) is a freshwater lake in the Sakha Republic (Yakutia), Russia.

The nearest inhabited place is Pokhodsk, Lower Kolyma District, located about 130 km (81 mi) to the southeast in a straight line.

[3] Bolshoye Morskoye, meaning "Big Sea", lies way north of the Arctic Circle, in the northern part of the Kolyma Lowland.

It belongs to the basin of the Bolshaya Chukochya river, which flows close to the lake to the south.

The main outflowing river is the 67 km (42 mi) long Ankavaam (Morskaya), a left tributary of the Bolshaya Chukochya.