The England international striker, 23, has been the subject of interest from a host of Premier League clubs after being told he was no longer wanted under new United boss Louis van Gaal.
Tottenham were seriously interested in signing Welbeck on loan, while Newcastle, Sunderland and Hull City also made enquiries, but Arsenal moved for the forward late on deadline day, asking for an extension to 1am to get the deal finalised.
Sky sources reported earlier on Monday that the player had left training with England in Hertfordshire to undergo talks with the Gunners and he is now set to sign a long-term deal at the Emirates Stadium.
Welbeck came through the youth ranks at United and rose to prominence on a successful season-long loan at Sunderland during the 2010-11 season.
He has scored 20 Premier League goals in 90 appearances and has won 26 caps for England since his 2011 debut, scoring eight.
However, his only start this season came in the League Cup thrashing at MK Dons last week – a game Spurs boss Mauricio Pochettino watched – and United have since struck a deal to sign Radamel Falcao on loan from Monaco.
Agree with you O.K.,a very bad move to let him go.He is a proper RED,who needed a better chance.The duo of Van Persie and Rooney is going to even cause Falcao to sit on the bench,so why did we bring him in anyway?Any way goodluck Danny,dont come and score against us though.- muforever001