William Troost-Ekong, a Super Eagles defender , will play in the English Premier League next season.
However, Watford defeated Millwall 1-0 thanks to Ismaila Sarr’s penalty in the 11th minute.
Brentford’s victory at Bournemouth earlier on Saturday, meant that only three points would be good enough for Xisco Munoz’s men to earn automatic promotion to the top flight.
The win took the Hornets’ tally in the Championship to 88 from 44 games.
Watford have now been promoted alongside with Norwich, who came down with them last season.
Troost-Ekong made a surprise move to join Watford in the English second-tier from Italian top-flight club Udinese last summer.