The deal was completed about two hours after Monday’s transfer deadline of 11pm, as Arsene Wenger scurried frantically to bolster his attacking line-up.
Welbeck, who can play across the front three, make 90 appearances for United and scored 20 goals. The 23-year-old has also won 25 caps for the England, since he made his senior debut in 2008.
According to a statement on Arsenal’s official website, “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”.