Sakhalin is an island in Northeast Asia. Its north coast lies 6.5 km (4.0 mi) off the southeastern coast of Khabarovsk Krai in Russia, while its southern tip lies 40 kilometres (25 mi) north of Japan's Hokkaido. A marginal island of the West Pacific, Sakhalin divides the Sea of Okhotsk to its east from the Sea of Japan to its southwest. It is administered as part of Sakhalin Oblast and is the largest island of Russia, with an area of 72,492 square kilometres (27,989 sq mi). The island has a population of roughly 500,000, the majority of whom are Russians. The indigenous peoples of the island are the Ainu, Oroks, and Nivkhs, who are now present in very small numbers.

The Sea of Japan (see below for other names) is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, it has almost no tides due to its nearly complete enclosure from the Pacific Ocean. This isolation also affects faunal diversity and salinity, both of which are lower than in the open ocean. The sea has no large islands, bays or capes. Its water balance is mostly determined by the inflow and outflow through the straits connecting it to the neighboring seas and the Pacific Ocean. Few rivers discharge into the sea and their total contribution to the water exchange is within 1%.