Koeman, who played for the LaLiga club between 1989 and 1995, returns to the Nou Camp to replace Quique Setien after his sacking in the wake of last week's humiliating 8-2 Champions League loss to Bayern Munich.
Here, the PA news agency looks at the highs and lows of the Dutchman's managerial career.
High – After spells as assistant manager for Holland and then Barcelona, Koeman helped set Vitesse Arnhem on course for UEFA Cup qualification during his maiden managerial role.
High – Koeman moved on to Ajax in December 2001 and ended the club's three-year wait for the Eredivisie title in his first season, clinching a domestic double. Ajax were again champions under Koeman in 2004.
Low – He took over Portuguese champions Benfica in 2005 but failed to mount a title defence as they finished third behind rivals Porto and Sporting Lisbon. However, the club did reach the Champions League quarter-finals, beating Manchester United and holders Liverpool en route.
High – After a single season in Lisbon, he returned home and guided PSV Eindhoven to the Dutch title and Champions League last eight.
Low – He won the Copa del Rey with Valencia in 2008 but was dismissed later that year after the Spanish club plummeted to just two points above the LaLiga relegation zone and finished bottom of their Champions League group.
Low – Koeman left reigning Eredivisie champions AZ Alkmaar in December 2009 following seven defeats in his first 16 league matches after succeeding Louis Van Gaal.
High – Following three seasons with Feyenoord, he guided Southampton to their highest Premier League finish of sixth in the 2015-16 campaign, securing Europa League qualification in the process.
Low – Koeman was sacked by Everton in October 2017 with the club languishing in the Premier League relegation zone following a summer of heavy spending. A forgettable tenure at Goodison Park lasted just over a year.
High – Taking over a team which had failed to reach the 2018 World Cup, Koeman led Holland to qualification for Euro 2020, as well as finishing runners-up in the inaugural Nations League.