The highly-rated player becomes the third signing of the summer, after the Hornets managed to capture Jerome Sinclair from Liverpool and Christian Kabasele from Genk.
Granada’s Player of the Year last season
Success can play in a wider position or through the middle and attracted interest from all over Europe, but the Hornets managed to capture his signature.
The forward made the move up from Granada B to the first-team in the 2014-15 season and never looked back, having scored six goals in 30 La Liga games last season.
His goals helped the Spanish outfit narrowly avoid relegation, and in the process, Success picked up the Player of the Year award for his impressive performances throughout the season.
Success has player for Nigeria at Under-17 and Under-20 level, scoring nine goals in 11 appearances combined, and looks set for a bright future.
Big signings confirmed for the Hornets
The Success deal was one of a number of deal to have been completed by the Hornets during a momentous day, with it being the fourth anniversary with the Pozzo family in charge.
As well as the captain signing a new deal and the two transfers being announced, it was revealed that Watford turned down an astonishing £37.5 million bid from Shanghai SPIG for forward Odion Ighalo.
In terms of players leaving, Gabriele Angella has moved to Udinese and Joel Ekstrand has been released by the club.