The 24-year-old joined Forest from relegated Watford on a permanent transfer.
It was gathered that the deal willl run for four years.
According to transfer market expert, Fabrizio Romano, the total fee is around 20 million pounds.
“Official and confirmed. Emmanuel Dennis joins Nottingham Forest on permanent deal from Watford, four year deal. #NFFC
Total fee will be around £20m,” Romano tweeted on Saturday night.
With the development, Nottingham Forest now has two Nigerian players. Super Eagles forward, Taiwo Awoniyi arrived at the club earlier this summer from German outfit, Union Berlin.