The 19-year-old was a first team player in Arsene Wenger's set-up and even made two Europa League appearances for the Gunners this season.
"FC Barcelona and Arsenal have come to an agreement by which the midfielder Marcus McGuane will be transferred to Barca B," a statement from Barcelona read.
"McGuane’s deal will keep him at Barca for three seasons, plus two option years. The buyout clause has been set at €25m.
"McGuane passed his physical with the FC Barcelona medical services team and formalized the deal on Tuesday afternoon at the Club’s offices at the Ciutat Esportiva. On hand for the signing were the Football Manager, Pep Segura, and the Technical Secretary for Professional Academy Football, Jose Mari Bakero," the statement added.