Nidelva is the main river in the Arendal drainage basin in Agder county, Norway.

The watershed covers 4,011 square kilometres (1,549 sq mi) and has an average waterflow of 114.4 cubic metres per second (4,040 cu ft/s) at Rykene near the mouth.

The river empties into the Skaggerak just northeast of Rykene in the town of Arendal between the mainland and the island of Hisøya.

[1] There are sixteen hydro-electric power stations are built along the river, making this one of Norway's most controlled drainage systems.

[1] Salmon can be found from the mouth to as far as 22 kilometres (14 mi) up the river, where the Evenstad power station is located, effectively blocking their path further upstream.