Sisak Cathedral was built in the 18th century and dedicated in 1765. The spire was built in 1760.
After an earthquake in 1909, the old Baroque facade was replaced with a Neoclassical facade, with details in the Art Nouveau style. The church was also damaged during the Croatian War of Independence.
On 5 December 2009 Pope Benedict XVI established the Diocese of Sisak. As a result, the parish church became a cathedral.
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The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.