The Tricky Trees had already broken their transfer record earlier this summer when they signed Awoniyi from Union Berlin for £18.45m. However, they have now broken the record in just one month with the signing of another Nigerian international.
A statement on the club's website read: Nottingham Forest is delighted to confirm the signing of striker Emmanuel Dennis from Watford.
The 24-year-old forward arrives on Trentside having hit double figures for The Hornets last season, returning 10 goals from 33 Premier League starts.
The 24-year-old joins Steve Cooper's men after just a year at Watford. Dennis joined The Hornets from Club Brugge for £3.5m but has left following an impressive debut campaign in the Premier League.
Although his performances could not save Watford from the drop, Dennis was arguably the best player for The Hornets, scoring ten goals and assisting another six in the English top flight.
Following Watford's relegation, he was linked immediately with a move away from the club. Everton and Newcastle were initially credited with an interest in the Nigerian international's service before Forest swooped in.
Dennis could make his debut against The Hammers
Depending on if he was registered on time, Dennis could make his debut under Steve Cooper in Forest's home clash against West Ham on Sunday afternoon.
However, if his registration was not certified before Friday's deadline, Dennis would have to wait until next Saturday against Everton for his Forest debut.
The former Club Brugge man has already played two league games with Watford in the Skybet Championship this season.