The Hornets have signed 21-year-old Senegal international for an undisclosed fee, which is understood to have broken their club transfer record.
Sarr, who joins from Ligue 1 side Rennes, has signed a five-year contract at Vicarage Road.
The Senegal international started his club career at Metz. He made 32 appearances for Rennes last season and scored eight goals.
“Hopefully the Watford fans are going to see some goals and the way I play with a lot of dribbles and speed, and hopefully that’s what they will be seeing a lot of in the future,” Sarr said in an interview.
“I will give my best for the fans and for the club and they will see every week that I will give 100 per cent for Watford.”
“It’s a real pleasure to join this club. It was my dream to play in England and I’m really excited to be here and to have signed for Watford,” he said.
“It was a dream for many years to play in this country because of the intensity and speed of the game. That is the way I play so it’s perfect for me.”
The wide man also played for his country in the AFCON this summer, helping them reach the final where they lost to Algeria.
Watford have also confirmed that Sarr will wear the No. 23 shirt this season. The Hornets begin their Premier League campaign at home to Brighton on Saturday.