The north London club confirmed the acquisition of Welbeck via its official Twitter account:
"It's official. Arsenal have signed Danny Welbeck from Manchester United," the club tweeted.
Welbeck, 23, has made 26 international appearances for England and joined United's academy at eight years old and scored 29 goals in 142 appearances for the Red Devils.
Welbeck will fill a void up top for the Gunners, who lost striker Olivier Giroud to a broken leg against Everton on Aug. 23.