Riewoldt has stepped down after 11 years in the role.

Geary, recruited through the Saints' rookie list in 2006, has played 141 games since his 2008 debut.

The 28-year-old joins a select group of former rookie-listed players who have risen to the captaincy, including Sydney's Kieren Jack, Collingwood's Nick Maxwell and Matthew Boyd from the Western Bulldogs.

Geary will head a new-look player leadership group at St Kilda, who have the makings of a team on the cusp of a new era.

They have not played finals since 2011, but only finished outside the top eight last season on percentage after winning six of their last eight matches.

Apart from Riewoldt, fellow veterans Leigh Montagna and Sean Dempster have also stood down from official leadership duties.

Josh Bruce, Seb Ross and Dylan Roberton are in the leadership group for the first time, joining fellow deputies David Armitage, Jack Newnes, Jack Steven and Mav Weller.

Geary said in a club statement that the captaincy is an incredible honour.

"St Kilda is a proud and historic club with a really bright future ahead of it and I am humbled and excited to take on this responsibility.

"I have learnt so much under outstanding leaders like Roo and Joey (Riewoldt and Montagna) and I can't wait to lead this playing group."

Coach Alan Richardson said Geary, who finished runner-up in last year's club best and fairest voting behind Steven, was the overwhelming choice as their new captain.

"Jarryn's a selfless team man who speaks through action and his display of leadership both on and off the field is exemplary," Richardson said.

"He demands a lot of himself and challenges those around him to be at their best."

Along with congratulating Geary, Saints president Peter Summers praised Riewoldt for his "outstanding contribution as club captain and his selfless act of stepping aside from the leadership position, which has created the opportunity for Jarryn to lead the group into the next exciting chapter of our club's history."