Saints have confirmed that Calum Chambers has completed his move to Arsenal.
The 19-year-old right back is believed to have moved for a fee of around £12m up front, rising to £16m.
Petersfield-born Chambers, who has been with the club since the age of seven, was under contract at St Mary’s until 2017, having signed a new four-year deal last summer.
The teenager made his Premier League debut on the opening day of last season, helping Saints to a 1-0 win at West Brom.
England under-19 international Chambers went on to make 18 starts in the competition and four substitute appearances.
Saints will rely on Nathaniel Clyne to assume right back duties.
Speaking about the deal before confirmation Arsenal boss Arsene Wenger said: "He can play at centre-back, right-back and central midfield. I hope he will give us competition for the players in these three positions.
"He hasn't played many games, no, but the English players on the market in England are very expensive and, at the end of the day, I was ready to take a gamble because he is a player for the future."
He later added: "We are very pleased that Calum has agreed to join us. The way he adapted to the Premier League last season with Southampton at a young age shows that he has tremendous quality. 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."
Chambers gave an interview to the official Arsenal website after penning his new deal and said: "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. I'm looking forward to joining up with my new team-mates today and beginning preparation for the season ahead.
"I would like to take this opportunity to say thank you to everyone at Southampton FC for everything they have done for me and for making this move possible."