The Free City of Danzig (German: Freie Stadt Danzig; Polish: Wolne Miasto Gdansk) was an autonomous city-state established on January 10, 1920. It was established in accordance with the terms of the Treaty of Versailles of 1919, which placed the Baltic port city of Gdansk under League of Nations protection, with special economic-related rights reserved for Poland. The Free City was represented abroad by Poland and was in a customs union with it. The railway line that connected the Free City with Poland was administered by Poland. Similarly, the separated military post within the city's harbour was also given to Poland. There were also two post-offices, one municipal, the other Polish-run.
A Polish post office operated from 1925 in the harbour of Danzig, from which stamps, destined for Poland, were issued. Most of these stamps were existing Polish postage stamps overprinted "PORT GDANSK". The office remained open until the outbreak of World War II.