The 29-year-old was released by Watford earlier this month following their relegation from the Premier League and Brighton have pounced to bring him in on a one-year deal.
Welbeck, who has 42 England caps to his name, was at Watford for just one season, scoring three goals in 20 appearances across all competitions.
Across his Premier League career at United, Arsenal, Watford and a loan spell with Sunderland, Welbeck has scored 44 Premier League goals in 224 appearances.
‘We are delighted to have Danny joining us. He will complement our existing striking options,’ said Brighton manager Graeme Potter.
‘He’s got an undoubted pedigree, and he is quick and technically very good. He will add to our attacking threat and we are thrilled to add another player of his experience and quality.’
‘Watford FC confirms that it has reached an agreement to allow Danny Welbeck to leave on a free transfer,’ the statement read.
‘Welbeck featured 20 times for the Hornets last season, scoring three goals.
‘We wish Danny every success in the future.’
Brighton’s technical director Dan Ashworth said of the signing: ‘I think Danny’s arrival concludes what we feel has been a very good window for the club.
‘A lot of planning, diligence and hard work went into the recruitment side, and I would like to thank all the club’s different departments for their efforts in achieving that.’
The Seagulls take on Crystal Palace on Sunday, but Welbeck will be available to make his debut for the club in their next game, at home against West Brom on 26 October.