The Irish striker has put pen to paper on a new four-year contract.
The Republic of Ireland forward scored 13 goals in 34 appearances for the Saints last season despite often having to fight with Italy striker Graziano Pelle for the lone frontman role at St. Mary's.
The 29-year-old's impressive form - his most prolific individual campaign in five years - had seen him linked with a move away this summer, with clubs such as Liverpool having been reported as interested earlier this summer.
However, he has confirmed that he has instead committed his future to Southampton by signing a deal which keeps him tied to the club until 2020, when he will be 33.
Long tweeted: "Delighted to have renewed my contract. Thanks to the club & the fans for your support!"
The news is a huge boost for the Saints, particularly as they have lost both Pelle to a lucrative Chinese Super League move, and forward Sadio Mane in a big-money deal to Liverpool.
Long will now have his sights set on establishing himself as the undisputed first-choice centre-forward for the club as the Saints begin life under Claude Puel next month.
Reacting to the news on Twitter, Southampton fans were universal in their delight at the news as one of their prize assets committed his future.
Congratulations on the new contract @ShaneLong7 . Thoroughly deserved.— Jamie Kemble (@jamiekemble) July 21, 2016
Good bit of business that tiring Shane Long down for another 4 years. Hopefully he can carry his scoring run from the summer to the PL— TheMoanyOne (@TheMoanyOne) July 21, 2016
Shane Long staying is brilliant! One of my favourite #saintsfc players from over the years— Phil Smith (@PSmith1994) July 21, 2016