England defender Luke Shaw has joined Manchester United from Southampton for £27m, making him one of the world's most expensive teenagers.
The 18-year-old's fee could rise to £31m, depending on his performance. An initial £27m offer was rejected in May.
The left-back has agreed a four-year contract with United, with an option to extend for a further year.
"I want to continue to progress my career and joining United is the ideal place for me to do that," he said.
Ryan Giggs, United's assistant manager, added: "Luke is a very talented young left back with great potential.
"He has developed immensely during his time at Southampton and has all the attributes to become a top player."
Shaw told Saints at the end of last season that he wanted to join United.
He became the youngest player to feature at the 2014 Fifa World Cup in Brazil when he played the entire 90 minutes of England's 0-0 draw with Costa Rica on Tuesday.
He is United's second summer recruit - and second in as many days - after they signed Spain midfielder Ander Herrera, 24, from Athletic Bilbao for a fee believed to be £29m.
Manchester United's Wayne Rooney was the world's most expensive teenager when he moved to Old Trafford in September 2004 for an initial fee of £20m.
Shaw's signing eclipses that and matches the £27m Paris St-Germain paid Roma for 19-year-old Marquinhos in July.
The youngster also becomes the fourth most expensive defender in world football.
|World's most expensive defenders|
|David Luiz||Chelsea||Paris St-Germain||40||2014|
|Thiago Silva||AC Milan||Paris St-Germain||36||2012|
|Rio Ferdinand||Leeds United||Manchester United||33.1||2002|
|Luke Shaw||Southampton||Manchester United||27||2014|
|Fabio Coentrao||Benfica||Real Madrid||27||2011|
Southampton insist the money they have received from United will be reinvested in the team.
"While we are sad to see Luke depart the club, we fought to ensure that we got the right deal for a player in whom we have invested a great deal of work over the past decade," said executive director Les Reed.
"This deal is a good one for Southampton Football Club and hopefully sends a clear message to other clubs wishing to bid for our players.
"Luke's transfer fee will be reinvested into the team as our new manager, Ronald Koeman, builds for the new season and for years to come."
Meanwhile, Saints midfielder Adam Lallana is set to have a medical ahead of a proposed £25m move to Liverpool.
The 26-year-old England international is expected to complete his transfer within the next 24 hours.
England striker Rickie Lambert has already moved from Saints to Anfield.