Gdynia, Poland, a city and a major seaport on the Gulf of Gdańsk, an arm of the Baltic Sea. Gdynia is in northern Poland just northwest of the port of Gdańsk. Together the two cities handle most of Poland's foreign trade. Gdynia is also a naval base and a manufacturing city with shipbuilding and ship-repairing facilities. Gdynia was a fishing village until after World War I, when the Polish government developed it as a port.