The 26-year-old has been a revelation across half-back in his seventh year with the Saints, averaging 21 disposals, four marks and two tackles per game.
The new role for Sinclair has seen his game taken to a new level in 2021 as the Saints look to secure back-to-back finals finishes.
St Kilda list boss James Gallagher stated his delight to have the dashing defender recommit to the club.
"We've been really pleased with Jack's performances this season, so it's great to have him recommit to the club", he said.
"His speed, skill and running power allows him to play a number of different positions for us and his form this year has been exceptional.
"It has also been great seeing him take on more ownership of our group off the field, finding his voice as an emerging leader among the players".
Sinclair, a life-long St Kilda fan, expressed his joy in remaining a Saint.
"I'm really excited to stay at the club and play a part in what we're building", Sinclair said.
"I've loved this club for as long as I can remember, so to know I'll be playing here for the next few years is awesome".
"We're a really close and connected group and I believe the best is yet to come".
Sinclair was selected by St Kilda with pick No. 1 in the 2015 Rookie Draft, becoming a draft steal for the Saints.
Across his 115 career games, Sinclair has been moved across all lines of the field, which included an 18-goal debut season.