The Chapel of St Casimir is a particularly holy site in Vilnius Cathedral, and a special chapel that continues to attract tourists and pilgrims. Casimir (1458–1484) was the second son of Grand Duke Casimir of Lithuania (ruled 1440–1492). He achieved fame both for his ruling skills and for his pious way of life. He died very young, and was buried in Vilnius Cathedral, where believers soon started to witness various miracles. The canonisation process started not long after his death, but it remained unfinished throughout the 16th century. The prince was officially acknowledged as a saint in the early 17th century, a fact celebrated with huge festivities in Vilnius in 1604.

The present chapel was built in 1623–1636, at the initiative of Sigismund III Vasa, the grandson of St Casimir’s brother, and the great grandson Ladislaus IV Vasa. It was designed and built by the Italian architects Matteo Castello and Costante Tencalla. The chapel was the first building in the Grand Duchy of Lithuania to be decorated mainly with marble. Its exterior was embellished with slabs of sandstone, which were rare here. The prince’s remains were transferred to the chapel in 1636. In the late 17th century, the dome and the altar were adorned with magnificent works of stucco.