The Hornets headed into Saturday’s Championship clash sitting on 85 points, seven clear of third-placed Brentford with three games left to play.
Watford knew a victory would guarantee promotion back to the Premier League, and Ismaila Sarr’s penalty ensured Xisco Munoz’s side will finish second at the very least.
The Hornets trail leaders Norwich by five points with two matches remaining in the race to be crowned champions, but Munoz and his players did not care much about that as they celebrated wildly at full-time of Saturday’s win.
Munoz was carried aloft by Watford players and staff in the spring sunshine, while champagne was popped as the squad celebrated the prospect of next season back in the Premier League.
Norwich and Watford will be joined by one of Brentford, Bournemouth, Barnsley, Swansea and Reading, though The Royals – who sit seventh – will require results to go their way elsewhere to make the playoffs.