Duchy of Łęczyca (Polish: Księstwo łęczyckie) was one of the duchies of Poland. It was created around 1231 from Province of Łęczyca, a part of the Seniorate Province by Konrad I of Masovia. Province of Łęczyca existed since turn of the 11th/12th century.
In 1264 the Duchy of Sieradz was split from it. The last duke of this duchy was Władysław the Hunchback. After his death in 1352 the duchy became part of the Kingdom of Poland and was reorganized into the Łęczyca Voivodeship.