It was the northern half of East Prussia (formerly part of Germany).

When the USSR invaded formerly German or German occupied Eastern Europe at the end of WW2, they basically called the shots when it came to borders, and they wanted that area because it was a strategic military port.

East Prussia was formerly mainly populated by Germans, but like most Germans living in Eastern Europe after WW2, they were deported to what remained of Germany. The southern half of East Prussia was given to Poland, and the northern half to the USSR. It was administratively part of the Russian SSR and Russians were moved in to populate the area.