The outside back will move to union from October 2024 until at least the end of 2027 World Cup in a major boost for Rugby Australia before they host a British and Irish Lions tour in 2025 and the global showpiece two years later.
"Welcome back to rugby, Joseph," Rugby Australia chairman Hamish McLennan said in a brief statement. The governing body did not disclose details of the contract.
Suaalii played union at school but switched to league with the Sydney Roosters in 2021 and has since established himself as one of the best young players in the NRL. He will leave Roosters when his contract expires at the end of the season, the club said.
"Joseph has been transparent with the club and we respect his decision," Roosters head coach Trent Robinson said.
Suaalii will be the highest profile league player to move to union since fullback Israel Folau, who arrived via Australian Rules and made a huge impression in his first Wallabies tests against the Lions in 2013.
Suaalii, who was born in Sydney but represented Samoa at last year's Rugby League World Cup, has all the skill, pace and power to match Folau's impact on the 15-aside game.
"The Sydney Roosters have been great to me and my family since I got here and the club is always going to be a big part of me," Suaalii said. "There's still a long way to go in my journey with the Roosters."
Australia coach Eddie Jones said when he returned to the job in January that he was looking to reverse the talent drain from union and bring back former schoolboy internationals who had defected to rugby league.
In his first stint as Australia coach from 2001 to 2005, Jones brought in backs Wendell Sailor, Lote Tuqiri and Mat Rogers from rugby league in a high profile recruitment drive.
Rugby union has fallen on hard times in Australia in recent years, limiting its attraction to talented youngsters, but Rugby Australia have been banking on the lure of a Lions tour in 2015 and a home World Cup two years later.
