Lonja is a river in central Croatia, a left tributary of Sava. It is 133 kilometres (83 mi) long and its basin covers an area of 5,944 square kilometres (2,295 sq mi).
Lonja rises in the Kalnik mountain in northern Croatia, southeast of Novi Marof, at 46°07′57″N 16°22′17″E / 46.132404°N 16.371417°E. It flows westward until turning south near Breznički Hum, passing east of Sveti Ivan Zelina, and turning southeast near Sveta Helena. East of Lupoglav it turns south again, passing through Ivanić-Grad and nearing the river Sava. It then flows in parallel to Sava for the rest of its course, and the nature park Lonjsko polje, a protected area, covers the remainder of the Lonja river basin. Near the end of its course, the river splits into Stara Lonja ("Old Lonja") that enters Sava at the eponymous village of Lonja; and Trebeš or Trebež that discharges into Sava some 5,5 km downstream in the eponymous village of Trebež at 45°20′53″N 16°46′24″E / 45.348056°N 16.773333°E.
The main tributaries of Lonja are: