Kotlin is a Russian island, located near the head of the Gulf of Finland, 32 kilometres (20 mi) west of Saint Petersburg in the Baltic Sea. Kotlin separates the Neva Bay from the rest of the gulf. The fortified city of Kronstadt is located on the island and forms part of a World Heritage Site that is Saint Petersburg and Related Groups of Monuments. The island serves as a gateway to Saint Petersburg and as such has been the site of several military engagements.

The Gulf of Finland is the easternmost arm of the Baltic Sea. It extends between Finland to the north and Estonia to the south, to Saint Petersburg in Russia to the east, where the river Neva drains into it. Other major cities around the gulf include Helsinki and Tallinn. The eastern parts of the Gulf of Finland belong to Russia, and some of Russia's most important oil harbors are located farthest in, near Saint Petersburg. As the seaway to Saint Petersburg, the Gulf of Finland has been and continues to be of considerable strategic importance to Russia. Some of the environmental problems affecting the Baltic Sea are at their most pronounced in the shallow gulf. Proposals for a tunnel through the gulf have been made.