The striker had been without a club following his release by the Gunners at the end of last season, but he has now arrived at Vicarage Road ahead of the new campaign.
The 28-year-old, who has won 42 England caps, has seen his career hindered by injury, but he is fit again after breaking his ankle before Christmas.
That injury was picked up against Sporting Lisbon in the Europa League and turned out to be the last of his 126 appearances in an Arsenal shirt.
Welbeck, who left Manchester United to join Arsenal in 2014, will challenge Troy Deeney, Gerard Deulofeu and Andre Gray for a place in Javi Gracia’s side.
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Welbeck, who was considered a fan favourite at both his former clubs, doesn't have a particular good scoring record for a striker, but his work rate for the team is second to none.
In 206 Premier League appearances, the England international has 42 goals and has provided 20 assists.
He averages 1.8 shots per game, whilst averaging 18 passes per game.
Whether he will be a starter for the London club is unknown, but he is certainly a good addition to the Hornets' squad.News Now - Sport News