The Gulf of Finland (Soome laht; Suomenlahti; Фи́нский зали́в; Finska viken) 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 (including Primorsk). 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.
The gulf has an area of . The length (from the Hanko Peninsula to Saint Petersburg) is and the width varies from near the entrance to on the meridian of Moshchny Island