Virgil van Dijk has impressed this season with a succession of imposing displays.
The 24-year-old's new deal is set to expire in the summer of 2022.
In an official club statement, executive director of football Les Reed spoke of the importance of tying down the Dutch international:
“Although Virgil only joined us in September we felt it was important to strengthen our commitment and we believe this also provides further indication of how serious our ambitions are for the coming years.”
The towering defender has proved to be a vital component in the Saints' impressive campaign, making 36 appearances, and helping the south coast club to the sixth best defensive record in the Premier League.
Many pundits predicted that the step up from the Scottish Premiership would highlight flaws in the Dutchman's game, but van Dijk has proved the sceptics wrong, forging a fantastic partnership with Jose Fonte at the heart of the Saints defence.
Southampton travel to Tottenham Hotspur tomorrow as they continue to push for a Europa League spot, before their final day showdown with Crystal Palace at St Mary's. They head into the final two fixtures high on confidence after successive victories against Aston Villa and Manchester City.