Sean Long says the opportunity to join Harlequins was one he could not let pass him by.
The legendary former Saints half-back will leave his position as assistant coach at the club to take up a role with the rugby union Premiership side.
Long said: “I have had 18 memorable years at St Helens both as a player and as a coach.
“Working under Justin Holbrook has been a pleasure, but I have always had a desire to work in rugby union and this opportunity at Harlequins is one I cannot let pass me by.
“I want to pay thanks to the whole club, to Eamonn, Mike, Justin, all the players and of course the fans for everything they have done for me.
“I wish the club all the success this season and beyond and I will always be a Saint at heart.”