The 21-year-old penned a five-year contract with the Hornets from French side Rennes, just a few hours before the English transfer window shut down.
Sarr was part of the Senegalese team that finished second in the 2019 Africa Cup of Nations in Egypt last month.
The youngster is Watford’s sixth summer signing behind Craig Dawson, João Pedro, Tom Dele-Bashiru, Sam Dalby and Danny Welbeck.
Sarr played 77 times during his two year stay at Roazhon Park, scoring 18 goals. His performances in the 2018/2019 Europa League season caught the eye of various clubs across Europe, which sought to acquire his services and he chose Watford.
The Premier League season begins on Friday 9 August with a game pitting last season’s runner’s up Liverpool against newcomers Norwich City.
Watford will play their season opener against Brighton and Hove Albion on Saturday.