The Cathedral of the Immaculate Conception is the most architecturally significant religious building in Siedlce, with a Baroque interior that rewards a look inside. The cathedral sits near the center of the historic district and has been the seat of the Diocese of Siedlce since the 19th century.
The interior features period altarpieces, carved woodwork, and the kind of accumulated ecclesiastical decoration that accumulates in a working cathedral over two centuries. The building survived World War II in better condition than much of Siedlce and remains the most intact historic interior in the city.