Rising midfielder Jack Sinclair will be a Saint for two more years as the club builds towards another premiership push.
The 22-year-old was due to come out-of-contract at the end of this year.
He has played 38 career games and has not missed a match since he was recalled for St Kilda's round six clash with Hawthorn this season.
The third-year player spent his first two seasons at the club as a forward but credits a move to the wing for his impressive form and contract extension.
This year he has averaged 21 disposals per game, up from a career average of 11.67 and has become a key cog in the Saints' midfield.
He admitted he suffered from a case of second-year blues only able to play half as many games as in his debut season but has repaid the club's faith so far in 2017.
St Kilda chief operating officer Ameet Bains said while the club is pleased with Sinclair's improvement, the coaching staff hope plenty more is to come.
"With less than 40 games to his name we know there is plenty of improvement in him still," Baines said.
"We look forward to watching him progress over the next two years."
With Sinclair locked-in, the Saints can now focus their efforts on re-signing big names like Dylan Roberton, Jack Billings, Shane Savage and Jack Lonie, as well as deciding whether to offer contracts to veterans Nick Riewoldt and Leigh Montagna.