The 38-year-old is one of the club's most popular figures, having enjoyed a 13-season career at Saints where he won two World Club Challenges, four Grand Finals and five Challenge Cups.
Following his departure for Hull FC in 2009, Long spent time coaching at Wigan Warriors before helping out in backroom roles at Salford Red Devils and Samoa.
However, he returns to Langtree Park as part of Keiron Cunningham's coaching setup, with the Saints head coach delighted to seal Long's signature.
"I am really happy that Sean has agreed to join my team. He is a legend at this club and will bring years of experience not only as a world class player but also as a first rate coach," Cunningham told the club's official website.
"He commands respect amongst the playing group and will complement our backroom team perfectly."
Former scrum-half Long - who scored 2,625 points in more than 330 games for Saints - added: "I am delighted to be back at a club I love so much.
"Saints are one of the greatest rugby league clubs in the world and to be part of the backroom team here was too good an opportunity to turn down. It was my chance to come home.
"The opportunity to work with Keiron Cunningham - a team-mate and friend who was and still is a legend at Saints - and the great squad the club has is very exciting.
"I am keen to develop my coaching career and working alongside Keiron, Jamahl Lolesi and Paul Wellens, as well as the rest of the squad, will enable me to do that."