Slavonia is, with Dalmatia, Croatia proper, and Istria, one of the four historical regions of Croatia. Located in the Pannonian Plain and taking up the east of the country, it roughly corresponds with five Croatian counties: Brod-Posavina, Osijek-Baranja, Požega-Slavonia, Virovitica-Podravina, and Vukovar-Syrmia, although the territory of the counties includes Baranya, and the definition of the western extent of Slavonia as a region varies. The counties cover 12,556 square kilometres or 22.2% of Croatia, inhabited by 806,192—18.8% of Croatia's population. The largest city in the region is Osijek, followed by Slavonski Brod and Vinkovci.

Brod-Posavina County is the southern Slavonian county in Croatia. Its center is the city of Slavonski Brod and it spreads along the left bank of the Sava river, hence the name Posavina. Other notable towns include Nova Gradiška.