Finland, Gulf of, an eastern arm of the Baltic Sea. It is about 260 miles (420 km) long (east to west) and 45 to 85 miles (72 to 137 km) wide. The gulf is bordered on the north by Finland, on the east by Russia, and on the south by Estonia. It is connected with Lake Ladoga (Russia) by the Neva River and with Lake Saimaa (Finland) by the Saimaa Canal. The most important cities on the gulf are Helsinki, Finland; St. Petersburg, Russia; and Tallinn, Estonia.

The eastern part of the gulf is frozen for three to four months during winter, but is cleared by icebreakers.