Sisak (sǐːsak; also known by other alternative names) is a city in central Croatia, spanning the confluence of the Kupa, Sava and Odra rivers, southeast of the Croatian capital Zagreb, and is usually considered to be where the Posavina (Sava basin) begins, with an elevation of 99 m. The city's total population in 2021 was 40,185 of which 27,886 live in the urban settlement (naselje).
Sisak is the administrative centre of the Sisak-Moslavina County, Croatia's biggest river port and a centre of river shipping industry (Dunavski Lloyd). It lies on the D36 state road and the Zagreb-Sisak-Novska railway. Sisak is a regional economic, cultural and historical center. The largest oil refinery in Croatia is here.
Prior to belonging to the Roman Empire, which gave it the Latin name Siscia, the region was Celtic and Illyrian and the city there was named Segestica or Segesta. Writers in Greek referred to the city as Siskía, Segésta, and Segestikḗ.
In German the