It flows through the Slovak town Rožňava and the Hungarian city Miskolc.

It is also known for the Battle of the Sajó River from 11 April 1241 between the Mongol Empire and the Kingdom of Hungary.

Catchment area of the Sajó river by country:[3] Important hydrological stations along the Sajó river:[3][2] (Rozsnyó) Complete list of streams (slovak 'potok'; Hungarian 'patak') and rivers flowing into the Sajó (from the estuary upwards):[2][4][1] tributary The origin of the name is the subject of scholar discussions.

[5] Newer theories associate the name with sió referring to fast streams.

The supporters argue by numerous Slavic toponyms in the river basin and by local names related to soľ (salt) and Moravia like Moravce, Soľár, Solišče, Soľka, Soľník, Morava (according to this interpretation the Hungarian name is a later translation).