Sisak is a city in central Croatia at the confluence of the Kupa and Sava rivers, 57 km southeast of Croatian capital Zagreb with an elevation of 99 m. Its population is 45,792.

Sisak is usually considered to be the place where the Posavina (Sava basin), begins and as it's Croatia's biggest river port since it is centre of river shipping industry (Dunavski Lloyd). Sisak lies on the main road Zagreb - Sisak - Petrinja (M12.2) and the railroad Zagreb - Sisak - Petrinja.

Chief occupations are farming, ferrous metallurgy (iron works), chemicals, leather (footwear), textiles and food processing plants as dairy products, alcoholic beverages), building material, crude oil refinery, thermal power plant.

The city hosts University of Zagreb's Faculty of Metallurgy and the department of the Nautical Faculty.

The long history of urban life (around 2,500 years) and the extremely favourable position on the confluence of three rivers, (the Sava, the Kupa and the Odra), have made Sisak a very important town in the history of Croatia. During the Roman Empire, a Christian martyr St. Quirinus was nearly killed in what was known at that time as Siscia. Today he is the patron saint of Sisak.

The triangular fortress of the Old Town from the 16th century, well-preserved and turned into the Native Museum, is the main destination of every tourist. The fortress is famous for the victory of the Croats over the Turks in 1593, what was the very first significant defeat of the up-to-then invincible Turkish army on European grounds. The Baroque palace of Mali Kaptol and the classicist ane named (Veliki Kaptol), the brick made Old Bridge or Stari Most over the Kupa, the ethnological park are most frequently visited objects in the town. Sisak has many rich mineral springs, (spa's) with healing properties in the temperature range from (42 to 54 °C).

Sports and recreation facilities in the town and the surroundings include mainly the waters and alluvial plains a public beach on the Kupa. All rivers, (Kupa, Odra, Sava with their backwaters offer fishing opportunities. There are hunting grounds in the regions of Turopolje and Posavina. Sisak is the starting point for sightseeing tours into Lonjsko Polje Nature Park.

During the World War II, Sisak was the site of a concentration camp which was part of the large Jasenovac cluster where Serbs, Roma, and Jews were killed.