The 17-year-old joined the Gunners for in excess of £12million.
“I’m so excited to be joining Arsenal. It’s a fantastic club and I’m really looking forward to the opportunity of playing at the highest level,” he said.
“Arsenal have a great manager in Arsene Wenger, top quality players and an attractive style of play.
“These were all important factors in me choosing to sign for Arsenal.
“I’m proud to be joining Arsenal but I would also like to thank everyone at Southampton for their help and support during the early part of my career, for which I will always be grateful.
“I’m now looking forward to giving absolutely everything in an Arsenal shirt.”
