Stevens copped a knock to the head against Brisbane in round one and was cleared to play against North Melbourne the following weekend but later reported concussion symptoms.

The 27-year-old has not played at any level since.

Stevens, who played 91 matches in nine seasons for West Coast, the Western Bulldogs and St Kilda, informed teammates on Monday of his decision to retire.

"This decision has taken me a long time to come to," Stevens said.

"After lengthy consultations with my family and on the advice of doctors, I've made the extremely hard decision that I cannot risk my health and future for the game.

"I'm so grateful for the support of the entire St Kilda football club, particularly the medical and player development staff who have been extremely supportive of me over the past few months."

AFL clubs have become increasingly cautious in their management of concussed players.

Former Western Bulldogs teammate Liam Picken has not played a game this year after copping a pre-season head knock while North Melbourne's Ben Jacobs has missed several games with delayed concussion symptoms.

Saints forward Paddy McCartin and Melbourne's Angus Brayshaw have also had well-documented struggles in recent years.

St Kilda coach Alan Richardson said the club would continue to support Stevens.

"He is a high quality person who is enormously respected by everyone at the Saints," Richardson said.

"When we were at our best last season he was a huge a driver of our midfield group, with his attack on the ball and the opposition.

"Ultimately Koby has had to make a decision for his future and we support him 100 per cent and wish him all the best for the future."

Australian Associated Press