The 35-year-old retired from playing in August after two years with Hull FC, and has become part of the coaching set-up at Salford City Reds.
But he is set to make his debut for the Grasshoppers against Hull Ionians on November 19.
He told the club website: "It's been an ambition of mine to play union and now seemed the right time to do it. Preston Grasshoppers has a great reputation as club and a team that produces quality players.
"Wade Dooley, Will Greenwood and Steve Borthwick are just a few of the players that have graced the first-team pitch and I'll be delighted to become someone who has joined this impressive list."
During his illustrious spell with St Helens, during which he won four Super League titles, half-back Long twice set the club record for the number of points in a season, racking up 284 in 1999 and 352 in 2000.
His defining performance in the international game came in Great Britain's 2006 Tri Nations victory over Australia in Sydney, setting up two tries in a 23-12 win, although he returned home from the tour shortly after due to "personal reasons".
Preston chairman Michael Bailey is delighted to have secured Long's services, with the player set to be formally unveiled on Thursday.
"Sean is undoubtedly a legend in rugby league and widely respected by fans of the union code. To have a player of his calibre in the team will bring a new found confidence to our players and bring additional skills and abilities to the team," Bailey said.
"Sean will be playing home and away games, which will help other clubs in our league since the RFU funding has been dented. We're delighted he's signed for us and welcome him to the Hoppers' fold."
The news of Long's move came as a shock to Salford football director Steve Simms, who said: "We're in discussions with Sean at the moment.
"We didn't know about it until yesterday. I think he just wants to have a bit of fun."
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