Saint Casimir Jagiellon (Polish: Kazimierz, Lithuanian: Kazimieras; October 3, 1458 – March 4, 1484) is a crown prince of the Kingdom of Poland and of the Grand Duchy of Lithuania who becomes a patron saint of Lithuania, Poland, and the young.

His feast day, the Saint Casimir's Day, is marked annually with Kaziuko mugė (a trade fair) held on the Sunday nearest to March 4, the anniversary of his death, in Vilnius.