Koeman returns to St Mary's at the weekend with Everton, and has made some shrewd signings as manager of both clubs.
During his two years at the Saints the Dutchman was very successful, and many of the players that he signed still play key roles in the current Southampton side.
He has also pulled off some great business since his move to Merseyside, and below is a combined XI of his signings at both clubs.
Goalkeeper - Fraser Forster (Southampton)
Forster was one of Koeman's first signings as Southampton boss. He arrived from Celtic for a fee of around £10m, and has become one of the most reliable goalkeepers in the Premier League.
The 28-year-old has also earned himself a spot in the England squad with his impressive performances.
Right back: Cedric Soares (Southampton)
The Portuguese full back signed from Sporting Lisbon for £4.7m in 2015, and had big shoes to fill following the departure of Nathaniel Clyne to Liverpool.
He has impressed since his move though, and also played an important role in Portugal's Euro 2016 victory in the summer after breaking into the team in the last 16 of the competition.
Centre back: Ashley Williams (Everton)
After John Stones's big money move to Manchester City, Koeman had to plug the gap that he left in the Everton back four. He did so by securing the signature of the experienced and reliable Ashley Williams for a very reasonable £12m.
Despite his pedigree though, Williams has struggled this season as part of a very shaky Everton defence, who have kept only three clean sheets so far.
Centre back: Virgil van Dijk (Southampton)
Another player that Koeman prised away from Celtic, van Dijk has been one of the Premier League's most commanding centre backs since his £13m move.
The 25-year-old Dutchman has formed a formidable defensive partnership with Jose Fonte, and his Saints form has been rewarded with a call up to the Netherlands squad.
Left back: Ryan Bertrand (Southampton)
The former Chelsea full back left the Bridge on loan in 2014 in search of first team minutes, and his successful spell under Koeman at Southampton was followed by a £10m permanent move to St Mary's.
Bertrand has since proved to the Blues that they were wrong to let him go, and has become a regular back-up for Tottenham's Danny Rose in the England set up.
Centre midfield: Oriol Romeu (Southampton)
Like Bertrand, Romeu left Chelsea after struggling for game time, and Southampton paid the Blues £5m for his services in 2015.
It was only when Victor Wanyama left for Tottenham this summer though that Romeu started to shine in a Saints jersey, and he is now showing his qualities under Claude Puel.
Centre midfield: Idrissa Gueye (Everton)
Arguably the bargain of this season, Gueye jumped ship from relegated Aston Villa to Everton in the summer of 2016 for a fee of £7.1m.
He has been an ever-present in the Toffees side this season, missing only one game - the 5-0 defeat away at Chelsea.
Attacking midfield: Dusan Tadic (Southampton)
The Serb was Koeman's first signing at Southampton in the summer of 2014, and has been one of the Saints key players since his £10.9m move from FC Twente.
He was originally signed to replace Adam Lallana, and has certainly provided the creative spark that the England midfielder did during his time at St Mary's.
Right wing: Sadio Mane (Southampton)
Although he has now moved on to Anfield, Mane was a huge success at Southampton after his £10m transfer from Red Bull Salzburg.
He scored 25 goals in 75 games for the club, and also broke Robbie Fowler's record for the quickest Premier League hat-trick, taking just two minutes and 56 seconds to net three times against Aston Villa.
Left wing: Yannick Bolasie (Everton)
Bolasie was Koeman's most expensive summer signing at Everton, costing the Toffees £28m from Crystal Palace.
It has proved to be a successful move, with the winger forming a fruitful partnership with striker Romelu Lukaku. Despite managing only one goal himself this season, he has provided four assists for his team-mates.
Striker: Charlie Austin (Southampton)
Koeman performed something of a transfer miracle by signing Charlie Austin for only £4m from Queens Park Rangers in January 2016.
A proven goalscorer, he almost repayed his entire transfer fee in his first appearance by coming off the bench to net at Old Trafford. Although injuries plagued the rest of his 2015-16 season, he has already scored eight goals in all competitions this campaign.