Arsenal have signed Southampton winger Alex Oxlade-Chamberlain for a fee reported to be worth £15m.
The 17-year-old has joined on a long-term contract, costing £12m up front with a further £3m in add-ons.
The England Under-21 international, hit 10 goals in 41 games last season and helped Southampton gain promotion from League One.
Gunners boss Arsene Wenger said: "We are looking forward to him fulfilling his huge potential with Arsenal."
Southampton suggested this was "one of the biggest deals in Championship history" and added that their previous transfer record - Theo Walcott's £12m move to Arsenal in 2006 - has been "blown out of the water".
"We are delighted that Alex has decided to join us," Wenger stated. "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.
"He is a perceptive passer of the ball, has a great understanding of the game and looks to be a very good team player."
Oxlade-Chamberlain, who turns 18 on 15 August, commented: "I'm so excited to be joining Arsenal. It's a fantastic club and I'm really looking forward to playing football at the highest level.
"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 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 made his Southampton debut as a 16-year-old in March 2010 and signed a three-year contract in August of that year.
He has made 44 first-team appearances in total and was included in the PFA League One Team of the Year last season.
Wenger is expected to further reinforce his squad before the transfer window closes on 1 September, but much will depend on the future of Cesc Fabregas, who has been linked with a move to Barcelona.
There is also speculation about midfielder Samir Nasri, while goalkeeper Manuel Almunia and striker Nicklas Bendtner are expected to depart.