"I'm delighted. I'm excited to be part of what could be a big future in the next four years for this," the Ireland international told the club's website (www.saintsfc.co.uk).
"I love it here, the set-up, the training ground, the lads as well – everyone around the club. It's a real family feel, and I feel like I'm a big part of that and I feel like I'm playing my best football here as well."
Long played all four games of Ireland's Euro 2016 campaign, where they lost to finalists France in the last 16.
Southampton continue to tie up key players on long-term deals after handing new contracts to Ryan Bertrand, Virgil van Dijk, Fraser Forster, James Ward-Prowse and Steven Davis in recent months.
(Reporting by Ian Rodricks in Bengaluru; Editing by Amlan Chakraborty)
