Arsenal have signed England Under-19 defender Calum Chambers from Southampton for about £16m.
The 19-year-old made 25 appearances for the Saints and Arsenal see him as an option at right-back, centre-back or in a more advanced role.
"The way he adapted to the Premier League last season at a young age shows that he has tremendous quality," said Gunners boss Arsene Wenger.
Arsenal have also enquired about Saints midfielder Morgan Schneiderlin, 24.
|Chambers for England in 2018?|
|In a BBC Sport feature on England's possible team for the 2018 World Cup, four of five national journalists questioned picked Chambers in defence.|
"He has a lot of the attributes that we look for in a young player and I am sure that he will do well with us," added Wenger.
Chambers flew out with the Arsenal squad - including Debuchy and Ospina - for their training camp in Austria on Monday.
Alex Oxlade-Chamberlain and Yaya Sanogo were also included after missing the New York trip at the weekend, while Sanchez is scheduled to arrive at the club's London Colney training ground on Tuesday.
"I am so happy to be signing for Arsenal. They are a team I have much admired for their playing style, and a team who have been one of the top sides in Europe for many years," Chambers told Arsenal's official website.
"I would like to take this opportunity to say thank you to everyone at Southampton for everything they have done for me and for making this move possible."
|From Saints to Gunners|
|Calum Chambers has become the third youth-team product to leave Southampton for Arsenal in recent years.||Alex Oxlade-Chamberlain followed Walcott to Arsenal in 2011 for £15m. The midfielder has made 66 appearances for Arsenal.|
|Theo Walcott left the Saints to join the Gunners in January 2006. He has since made 281 appearances for the club.||Walcott, Oxlade-Chamberlain and Chambers made 79 league appearances for Saints between them.|
"We are naturally disappointed whenever any of our academy graduates leave and that is very much the case with Calum's departure," Southampton executive director Les Reed told the club's website.
"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."
The arrival of Chambers at Arsenal could see right-back Carl Jenkinson, 22, leave the Gunners on loan to gain regular first-team football.
Gunners boss Wenger is still in the market for a holding midfielder and another defender - if captain Thomas Vermaelen leaves the club.