The 17-year old winger has joined the Gunners for a fee of £12million with £3million worth of add-ons according to Press Association Sport whilst Southampton's official website confirmed the youngster had left.
The statement read: "Alex Chamberlain has joined Premier League side Arsenal for a club record fee, subject to Premier League and FA approval.
"The teenager agreed personal terms and passed a medical this afternoon in one of the biggest deals in Championship history, which sees Saints' previous transfer records blown out of the water.
"The deal for the midfielder - who had made it clear he saw his future away from Southampton, was done with Saints in the driving seat for the duration of the protracted negotiations and the transfer was completed on our terms.
"The Club had long made it clear that it didn't wish to sell Chamberlain, and would only consider doing so should an offer meet our high valuation of the youngster, who turns 18 next week."