Koeman has today signed a three-year contract, and takes charge with immediate effect.
The appointment follows an extensive worldwide search for a new manager to move forward the vision and plan set out for the continued success and development of Southampton Football Club.
Koeman arrives at St Mary’s following a highly-successful spell in charge of Feyenoord, who he led to consecutive top-three finishes in the Eredivisie since joining the club in 2011.
Ronald is joined at Southampton by his elder brother, Erwin, who is appointed Assistant First Team Manager. Erwin has enjoyed a successful playing and coaching career of his own, having managed in the Eredivisie and taken charge of the Hungarian national team.
The pair will be joined by Jan Kluitenberg, who takes up the role of Assistant First Team Coach/Physical Coach. Kluitenberg worked with Ronald Koeman at Vitesse Arnhem, Benfica, AZ Alkmaar and Feyenoord.
Ronald, Erwin and Jan will be assisted by Saints' existing first-team support staff, while the club will be recruiting a high-level goalkeeping coach in the near future.
Les Reed, Executive Director at Southampton Football Club, said: “We are extremely pleased to have secured a new manager of Ronald Koeman’s calibre and status within the game.
“In appointing Ronald we have concluded our search for a manager who shares our values and vision for the future of Southampton Football Club.
“As part of our ongoing scouting process, we have been following the development of Ronald’s coaching career for some time and, through a more formal process this month, immediately identified him as our number-one choice.
“From the moment we met with Ronald, it was clear he had the vision and ambition to take our club forward, that he shared our football philosophy and could see the opportunity to move our club to the next level.
“The club, the players and the fans will all be excited to have a man of Ronald’s experience at the very highest level joining our club.
“Ronald’s passion for the opportunity here at St Mary’s was very clear, and we look forward to continuing our growth with him as part of our team.”
Koeman said of his appointment: “I am thrilled to be the manager of Southampton Football Club.
“From my very first meeting with Les Reed, I could see clearly that this was the right club for me. We share the same philosophy of football, and the same belief in how to develop the club.
“Southampton have a talented first-team squad, a world-class academy and the infrastructure to sustain success in the Premier League.
“The opportunity to continue the development of this famous football club was simply too good to turn down. I cannot wait to get started.”