Financial details were not disclosed, but British media reported the Saudi Pro League side paid £51 million for the England international, who had two years left on his Villa contract.
Watkins signed a three-year contract as Al Hilal acquired a proven goalscorer after he scored 21 goals in all competitions last season, including five goals in the Europa League as he helped deliver a European trophy to Unai Emery's side.
However, an exit was on the cards before the transfer window closed, with Emery revealing the striker had apologised for making himself unavailable for the club's Premier League opener.
Watkins scored 91 Premier League goals for Villa, a club record, and 108 in all competitions since arriving from Brentford for £28 million in 2020.
The striker is among a string of departures from Villa in the close season following the exits of key players such as Morgan Rogers and Emi Martinez (both Chelsea), Ezri Konsa (Arsenal) and Youri Tielemans (Manchester United).
As the summer transfer window ramps up, Flashscore has everything you need to keep up with breaking news, rumours and analysis. Follow our live transfer blog to follow the biggest moves, whilst expert Dean Jones will be on hand to provide the inside stories from around the world.