Also known as the 'Pearl of Sieraków', the Church of the Immaculate Conception of the Virgin Mary was built by the Opaliński noble family in the 16th century. Its design is attributed to Christopher Bonadura the Elder, an Italian architect who was based in the Greater Poland area. The Bernardine church and monastery, which shares the same site, was built between 1625 and 1639. The church is claimed to be the only building that survived a huge wildfire that effectively razed the town in the early 19th century. During rennovations in 1991, the painted caskets of several members of the Opaliński family were discovered in the crypt. These have since been relocated to Sieraków Castle and are on display for visitors.