Which gulf separates Sweden and Finland and is the northernmost part of the Baltic Sea?

The gulf separating Sweden and Finland and forming the northernmost arm of the Baltic Sea is the Gulf of Bothnia.

The Gulf of Bothnia lies between Finland’s western coast and Sweden’s eastern coast. It is a broad, elongated body of brackish water divided into the Bothnian Bay in the north, the Kvarken strait in the middle, and the Bothnian Sea in the south. The Bothnian Bay is therefore the northernmost part of the Gulf of Bothnia—not a competing name for the entire gulf.

The gulf is about 725 kilometres long and has an average depth of roughly 60 metres. Its northern waters receive large river inflows and are much less salty than the Baltic farther south. Winter ice can cover the gulf for several months, so icebreakers are important to shipping, especially around northern ports such as Luleå, Oulu, Tornio and Vaasa.

A striking geological feature is post-glacial rebound: land is still rising after being compressed by Ice Age glaciers. The shallow Kvarken may eventually rise above sea level, separating the northern basin from the rest of the gulf. The Gulf of Finland and Gulf of Riga are separate Baltic subdivisions, while the Gulf of Venice is in the Adriatic Sea.