United boss Louis van Gaal has been very active in the transfer window so far, with the Red Devils announcing the arrival of Bastian Schweinsteiger as the same time as they try to invigorate an ailing midfield.
French international Schneiderlin had been with Saints since making a move from Strasbourg at the age of 18.
The now 25-year-old midfielder was a constant fixture in the middle of the park for Southampton for seven years and comfortably made the transition from League One football all the way up to Premier League and international standard.
The official Southampton twitter account posted this tribute below:
This move comes a week after Southampton rejected a bid from Manchester United for Schneiderlin's services.
Southampton boss Ronald Koeman and Les Reed - Saints' Head of Football Development - had said Schneiderlin will not be sold for anything less than the club's £25million valuation of the player. United reportedly put in a bid of £20million a few weeks ago, which was promptly rejected.
Now, it would appear United have agreed to stump up the extra £5million cash for the Frenchman.
Southampton have already sold Nathaniel Clyne and lost out to Tottenham in trying to sign former loan star Toby Alderweireld. This is the latest, yet expected, transfer blow to Saints this summer.
Saints fans will be hoping a suitable replacement is already in the works. Dutch International midfielder Jordy Clasie is being heavily linked to play under Koeman's management again after they worked so well at Feyenoord in Holland's Eredivisie.