The transfer initially sees the midfielder join Barca on a three-year deal, with the option of an additional two years.
McGuane will begin his life in Spain with Barcelona's B side after the Catalans immediately transferred him to their second-tier outfit upon completing the signing. Barcelona B compete in Spain's Segunda division - their equivalent of the Championship.
McGuane, 18, joined Arsenal at under-six level and went on to establish himself as a first-choice midfielder in the under-23 side.
His performances caught the eye of first-team manager Arsene Wenger, who selected McGuane for two Europa League matches earlier this season.
Wenger was particularly impressed with McGuane's versatility, and said last year: "He is a defensive midfielder, and overall he can play as a wing-back or even a centre back in training."
McGuane, born in Greenwich, is also an England youth international and has represented his country at under-17, under-18 and under-19 level.
Following an illustrious line of former Gunners that includes Thierry Henry, Marc Overmars, and Cesc Fabregas, McGuane becomes the first player since Thomas Vermaelen four years ago to swap Arsenal for Barcelona.
Arsenal released a statement on their website saying: "We would like to thank Marcus for his contribution and wish him well at Barcelona."
On a busy day of transfer dealings at the Emirates, Arsenal also moved closer to selling Olivier Giroud to Chelsea in order to facilitate Pierre-Emerick Aubameyang's arrival from Borussia Dortmund. THINK YOUR FRIEND WOULD BE INTRESTED? SHARE THIS STORY USING ANY OF THE SHARE BUTTON BELOW ⬇ PLACE YOUR TEXT ADVERT BELOW:>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> ( FREE ) secret Fruits that Increased My Manhood size and Lasting Power in 7-days... CLICK HERE TO GET IT Warning To Men: This 3 Foods Is Killing Your Erection. Click Here To Know Them