The Blaugrana agreed a deal for the Belgium international with the Gunners on Friday, with the center half snubbing interest from Manchester United to seal a switch to La Liga.
Arsene Wenger had previously admitted that he would rather sell the 28-year-old to a foreign club than to a domestic rival and, after being given a straight choice between Old Trafford and Camp Nou, Vermaelen headed for Catalunya.
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United had long been on Vermaelen's tail, with talk of the club's interest first emerging during the World Cup.
New boss Louis van Gaal is on the look out for ball-playing center backs to fit into his 3-5-2 system and had sanctioned a move for the former Arsenal captain, who has fallen down the pecking order at the Emirates Stadium.
Per Mertesacker and Laurent Koscielny have forged an impressive central defensive partnership for the Gunners over the past 18 months, limiting Vermaelen to a supporting role.
Arsenal has signed Calum Chambers from Southampton this summer and the 19-year-old could now be installed as third-choice center back at the Emirates if no new faces arrive before the close of the transfer window.