Former Roma manager Rudi Garcia
That means Southampton must now look for a replacement, and a host of names are already being linked with moves to the South Coast as Koeman's successor.
Now, French newspaper L'Equipe report that Southampton have approached two French managers - but the first, Rudi Garcia, has rejected their approach.
Ex-Nice boss Claude Puel
Garcia, 52, won the Ligue 1 title with Lille back in 2011, and eventually earned a move to Roma in 2013, taking the Italian side to second place in Serie A in both 2014 and 2015.
Despite taking the club to the Champions League, some struggles this past season led to Garcia's departure in January 2016, and he's been out of work ever since - but seemingly won't be heading to Southampton having allegedly snubbed their offer.
However, another Frenchman is still in contention. The same report adds that Claude Puel has been targeted by the Saints, having left Nice at the end of last season.
54-year-old Puel - who was ironically replaced by Garcia at Lille in 2008 - left Nice after four years in charge, where he won 39% of his games with the French club.
Now a free agent, Puel may be looking for a swift return to management - and whilst he's allegedly been approached, whether he'll prove to be Southampton's first choice target remains to be seen.
Would you like to see Southampton appoint Puel?