Long, 44, who recently announced his departure from Harlequins rugby union club, will take over from James Webster as assistant to Rhinos head coach Richard Agar.
Leeds director of rugby Kevin Sinfield said: “Last winter Sean regularly joined us in pre-season and gave valuable insight to the group and especially our half-backs and it is great that we can now bring him in on a full-time basis.
“Sean has a great relationship with Richard Agar having played under him at Hull FC and he is also well known to our group of players.
“Because of the Covid protocols in place for next season, our current assistant coach James Webster is not allowed to be part of two clubs’ training environments so he will solely be at Featherstone Rovers as head coach.
“Sean is vastly experienced and importantly adds to our knowledge pool as a coaching team.
“In Jamie Jones-Buchanan we have one of the best up-and-coming thinkers on the defensive side of the game and Sean is very much the equal of that when it comes to attack plus Chev Walker will continue to do an outstanding job in helping our young players transition from the academy to first-team.”
Long, who was capped 21 times by his country, won every honour in the game during his 12 seasons with St Helens before launching his coaching career with Salford in 2011.
He returned to St Helens as assistant coach in 2014 before leaving five years later to join Quins.
Long said: “I am looking forward to working with Richard Agar again after our time at Hull.
“I have a massive amount of respect for Leeds as a club. They are one of biggest clubs in the world and, as soon as I got the call off Kev and Rich, it was a no-brainer to join the club.
“I am looking forward to working with the half-backs at the Rhinos. It is a massive part of what I do.
“As an ex-half-back, I want to pass on what I have learnt to them and hopefully we can all get to a better place as a team. Game management, the kicking game and reading defences is something that excites me.”
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