The 19 year-old joins the Gunners for an undisclosed fee, having expressed a desire to leave St Mary’s after making 25 appearances for the club.
Executive Director Les Reed commented: “We are naturally disappointed whenever any of our Academy graduates leave the club, and that is very much the case with Calum’s departure.
“Calum made it clear to us that he did not see his future at Southampton. As such, it was crucial that we were able to negotiate a substantial fee that also reflects the work our staff have put into Calum’s development since he joined the club at the age of eight.
“Our position remains unchanged in that we are determined to keep our best young players at the club, as is evidenced by the decision to award Harrison Reed a new long-term contract last week, adding to the list of committed young professionals like James Ward-Prowse, Sam Gallagher, Jack Stephens, Jordan Turnbull, Matt Targett, Sam McQueen and others who have seen plenty of first-team activity recently.
“We have an abundance of talent in the Academy coming through, and everybody at the club is continuing to work hard on bringing in quality new players as we build a team capable of taking us further forward in the coming seasons.
“It is important in the transfer window to remain calm, resilient and above all patient. We have targets, and we are working hard to get them across the line.”