The 17-year-old starlet, who is believed to have cost the Gunners around £10 million, made 44 appearances for the Saints in all competitions, after breaking onto the first-team scene in March 2010.
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Oxlade-Chamberlain follows in the footsteps of fellow forward Theo Walcott, who also joined the Emirates Stadium outfit from Southampton in 2006, and has since gone on to make more than 100 Premier League outings for Arsene Wenger's side.
"We are delighted that Alex has decided to join us," Wenger told the club's official website.
"He is an exciting young player who will provide us with creativity and offensive quality. Alex is a versatile player who can play in a number of positions. He can play as an attacking centre midfielder, or wide left and right.
"Alex is a perceptive passer of the ball and has a great understanding of the game and looks to be a very good team player.
"Alex has done extremely well during his development with Southampton and we are looking forward to him progressing even further and fulfilling his huge potential with Arsenal Football Club."
Oxlade-Chamberlain, who has represented his country at under-18, 19 and 21 levels, winning one cap for the latter, expressed his delight at sealing such a high-profile move to North London.
"I'm so excited to be joining Arsenal. It's a fantastic club and I'm really looking forward to the opportunity of playing football at the highest level," he said.
"Arsenal have a great manager in Arsène 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 take this opportunity 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 meeting all my new team-mates and giving absolutely everything in an Arsenal shirt."
Oxlade-Chamberlain joins former Lille forward Gervinho and Charlton Athletic youngster Carl Jenkinson in signing for the Gunners during the close-season.
