An outstanding work of early Baroque executed by the best artists of Western Europe and modelled on the Sigismund Chapel in Wawel Cathedral.

St Casimir's Chapel was built in 1623-1636
. The Chapel of St Casimir in Vilnius Cathedral was built between 1623-1636 on the initiative of kings Sigismund III and Ladislaus IV in memory of Prince Casimir, the son of Casimir IV Jagiellon and Elisabeth Rakuszanka (Habsburg). It is an outstanding work of the early Baroque executed by Italian builders. Its creators were inspired by the Sigismund Chapel at Wawel Cathedral, which had been built a century earlier, and the Roman Capella Sistina and Capella Paolina at Santa Maria Maggiore.

The builder of the Chapel of St Casimir was Constantino Tencalla. Destroyed by the Muscovites in 1655, the chapel was renovated in 1678-1692 with the help of Michelangelo Palloni (frescoes) and Giovanni Pietro Perti (stucco).

In the crypt under the chapel, King Alexander Jagiellon, Queen Barbara Radziwiłłówna and the heart of King Władysław IV were buried.

King Casimir (1458-1484), predicted by his father to be the heir to the throne, was elevated to the altars in 1602, and was soon recognised as the patron saint of the Grand Duchy of Lithuania. 4 March in the Catholic Church is the feast day of St Casimir. While writing about it, it is impossible not to mention Kaziuki, the indulgent fair held in Vilnius on 4 March. Its origins can be traced to the celebrations surrounding the transfer of St Casimir's relics to the chapel founded in his honour (1636).

Originally, Kaziuki was held in Cathedral Square, but in the early 20th century it was moved to Lukiskaya Square. After Lithuania regained its independence, the fair returned to in front of the Cathedral Square and is still an important festival for Vilnius residents today. Traditional Kaziuk delicacies include gingerbread hearts and smorgasbord bagels, named after the town of Smorgonie, 80 km to the east of Vilnius (today in Belarus).