The Marstrand Free Port (Swedish: Frihamnen Marstrand) was a largely autonomous island territory of Sweden, during the Gustavian Era of the late 18th century, which effectively functioned as a merchant republic. As a free port designed with inspiration from the Italian porto Franco and declared in 1775 by King Gustav III, it became an urban centre of commerce both legal and illegal, refuge of political and religious dissidents, and hideout of wanted criminals.