Skipper Riewoldt, 31, has signed on until the end of 2015, while Montagna, 30, has committed until the end of 2016.
When their new deals end, both will be almost 33.
The names of both St Kilda stalwarts were rumoured to have been mentioned during the AFL Gillette Trade Period.
"I'm certainly happy that I have committed to the Saints and now it guarantees that I will play the rest of my career at St Kilda," Riewoldt told saints.com.au.
"I'm really excited about it and it is great to be able to do it with one of my great mates as well. It is an exciting time at the club and I am now committed to aiding the transition of some of the younger players into playing senior football."
Montagna was also looking forward to leaving an imprint on the next generation of Saints.
"It means a lot that the club has shown real confidence in me and I feel that I have a lot of good footy left to play," he said.
"St Kilda is on an exciting path and both Nick and I are looking forward to being a part of a really positive future."
Riewoldt has kicked 570 goals in 259 games and has been captain in 160 of them, making him the second longest-serving captain in the club’s history behind Danny Frawley (177).
The champion forward has also won five club best and fairests and been selected in four All Australian teams, once as captain.
Midfielder Montagna has played 212 games, is a two-time All Australian and boasts five top-three finishes in the Saints club award, including third placing this year.
The Saints are being put through altitude training in Boulder, Colorado, where they will remain until December 10.
