Arsenal did not specify either the length of Welbeck’s contract or the fee that they paid to sign him, but British media reports said that he had cost around £16 million ($26.6 million, 20.2 million euros).
“Danny Welbeck has joined Arsenal for an undisclosed fee on a long-term deal,” Arsenal announced on their website two hours after the transfer window closed on Monday.
“The transfer is subject to the completion of regulatory formalities. Everyone at Arsenal welcomes Danny to the club and looks forward to his contribution in an Arsenal shirt this season.”
Welbeck, a graduate of United’s youth system, was allowed to leave Old Trafford as the 20-time English champions closed in on a loan move for Monaco’s Colombia striker Radamel Falcao.
United, who have also allowed Javier Hernandez to join Real Madrid on loan, sent the England international a good-luck message on their official Twitter account.
“Danny Welbeck has signed for Arsenal after 29 goals in 142 appearances for MUFC,” the club said. “Good luck for the future, Danny.”
Welbeck leaves United having played a part in their 2009 League Cup triumph and their last Premier League title success in 2012-13.
Arsenal’s need for attacking reinforcements was heightened recently when France international striker Olivier Giroud was ruled out for up to four months with a broken tibia.
Giroud’s injury left 21-year-old Frenchman Yaya Sanogo as the only fit striker at the club, although new £30 million signing Alexis Sanchez can also play through the centre.
Welbeck has won 26 England caps, scoring eight goals, and is in national coach Roy Hodgson’s squad for Wednesday’s friendly against Norway and next week’s opening 2016 European Championship qualifier in Switzerland.
He follows Chilean forward Sanchez, France right-back Mathieu Debuchy, English defender Calum Chambers, and Colombian goalkeeper David Ospina in joining Arsenal during the close season.
The lithe, fleet-footed forward, who can also play on the flanks, could make his debut for Arsene Wenger’s side in their home game against Premier League champions Manchester City on September 13.