Watford returned to the Premier League after just one season in the Championship after defeating Millwall 1-0 thanks to a first-half penalty by Ismaila Sarr to seal promotion on Saturday.
Sarr scored the only goal of the game in the 11th minute after being fouled in the box. The home side were very comfortable after that and Millwall rarely threatened going forward.
They are mathematically promoted as they are 2nd on 88 points.
Brentford are 10 points behind and have three games remaining so they cannot catch them.
Watford joins Norwich City back in the Premier league.