The former Great Britain international will work alongside Saints’ head of strength and conditioning Matt Daniels in 2015.
“Whilst it is difficult to walk away from the game in terms of playing, I’m delighted to take on this role at the club,” Gardner said.
“I was going to retire at the end of 2015 but the club offered me this role to move into and it made sense. In the end it was the best decision for me and my family.
“I am retiring with a heavy heart. I had 15 good seasons at the best club in the world and have fond memories not only here but with Barrow, Hull KR and Great Britain too.
“But the opportunity to remain at this great club was too good to turn down.
“Strength and conditioning has always been an area of the game I have a deep interest in. Over the years I have put in a lot of work both on and off the field and looked at the science of what we do as players to continue to perform at a high level.
“I am currently in the third year of a Sports Science degree at Loughborough University and this role will enable me to gain good insight and experience into this part of the game.
“I have worked with Keiron for a number of years. We have a similar ethos and work ethic. The fact he wanted me to take on this role gives me a lot of confidence.
“I’m looking forward to working with the players who have been part of this club’s success and taking that ethic and mentality to the younger players who are in the first team squad and coming through the system.
“I’d never say never about playing again but the focus for now is on this new role and I can’t wait to get started.”
Saints head coach Keiron Cunningham is delighted to welcome Gardner to his staff.
“We’re delighted Ade has agreed to join the backroom staff here at the club,” he said.
“He is a superb professional who will bring a wealth of knowledge to the role built up over several years as a top level rugby league player.
“His work ethic is second to none and he has always been one of the best trainers at the club.
“That will help us deliver what we want to achieve as it sets an example to the young players we have not only in the first team but throughout our academy structure too.
“The fact he is in his third year studying Sports Science at University will also bring a great deal of knowledge and experience to the squad.”