Manchester United have agreed to sell Angel Di Maria to Paris St Germain for 63million euros.
Schneiderlin will remain at St Mary's until the summer of 2017 after the club confirmed they had tied him down to a new deal on Monday evening.
The 24-year-old Frenchman has played an important role for the Saints in their first season back in the top flight, missing just one Barclays Premier League match and scoring four goals."I am a very happy man," Schneiderlin told Saints Player after completing the seal this evening.
"To commit my future to Southampton is a big thing because this club has big ambitions - and I have big ambitions as well.
"I wouldn't have signed this contract if the club didn't have a big project. When I spoke to the chairman and the manager, I knew they wanted to do something very good at this club and I want to be a part of that.
"My main focus now is to do a great job on the pitch so we achieve all of the ambitions that we have."
Schneiderlin joined Southampton in 2008 and has been at the heart of their rise from League One into the Premier League.
He has quickly adapted to life in the top-flight, with his 102 tackles the most in the league, while his goal tally of four is the same number he managed in his previous four seasons on the south coast.
That has been enough to convince new boss Mauricio Pochettino to label him one of the league's "most complete midfielders" and waste little time in ensuring he remains an integral part of his regime.
"I am delighted that the club has been able to extend Morgan's contract," Pochettino said.
"During my short time with Southampton, Morgan has shown what a capable player he is and has demonstrated his potential to become one of the Premier League's most complete midfielders.
"His work-rate and determination make him an integral part of how we play, and so I am pleased he has been rewarded with a new long-term deal.
"Morgan has played a major part in the journey this club has been on over the past four years, and I am sure he will continue to develop as a player as we move towards our long-term ambitions."