The long history of urban life here (around 2,500 years) as well the extremely favourable position on the confluence of three rivers, (the Sava, the Kupa and the Odra), have made Sisak an important town in the history of Croatia.
During the Roman Empire when Sisak was known as Siscia, Christian martyr Quirinus of Sescia was tortured and nearly killed during Diocletian's persecution of Christians. Legend has it that they tied him to a millstone and threw him into a river, but he freed himself from the weight, escaped and continued to preach his faith. Today he is the patron saint of Sisak.
Braslav of Pannonia reigned from Sisak until this last bastion of the Pannonians was invaded. According to Historia Salonitana maior, Duke Tomislav reclaimed it soon after.
The 16th century triangular fortress of the Old Town, well-preserved and turned into the Native Museum, is the main destination of every tourist. The fortress is famous for the victory of the joint forces of Croats and Carniolans (Slovenes) over the Turks in 1593, known as the Battle of Sisak. It was one of the early significant defeats of the up-to-then invincible Turkish army on European territory. The Croatian Ban Toma Baka? Erdedi who led the defense in this battle became famous throughout Europe.
The Baroque palace of Mali Kaptol, the classicist Veliki Kaptol, the brick Stari most ("Old Bridge") over the Kupa, and the ethnological park are the most frequently visited landmarks.
During World War II, Sisak was the site of the Sisak Children's Concentration Camp which was part of the large Jasenovac cluster.
Sisak suffered much damage during the Croatian War of Independence starting in 1991. While Sisak remained within Croatian hands, the territory immediately south of the city was controlled by Serbs. The threat to Sisak was removed in 1995 following Operation Storm. Between 1991 and 1992, 24 Serb civilians were murdered by the Croatian forces
