Kotlin (or Kettle; Finnish Retusaari) is a Russian island, located near the head of the Gulf of Finland, 20 miles west of Saint Petersburg in the Baltic Sea. The fortified town of Kronstadt is located on the island.

In general outline, the island forms an elongated triangle, 7½ miles in length by about 1 in breadth, with its base towards St Petersburg. The eastern or broad end is occupied by the town of Kronstadt, and shoals extend for a mile and a half from the western point of the island to the rock on which the Tolbaaken lighthouse is built.

The island thus divides the seaward approach to St Petersburg into two channels; that on the northern side is obstructed by shoals which extend across it from Kotlin to Lisiy Nos; the southern channel, the highway to the former capital, is narrowed by a spit which projects from opposite Lomonosov on the Russian mainland, and, lying close to Kronstadt, has been historically strongly guarded by batteries. The naval approach to Saint Petersburg was greatly facilitated by the construction in 1875-1885 of a canal, 23 ft. deep, through the shallows, whereas cars will soon be able to travel overland to the island by using the Saint Petersburg Dam from the north and south shores of the Gulf of Finland. Started in 1980, but delayed by political upheaval in the 1990s, the project is now scheduled to be completed in 2008.