Spencer Leniu is currently with Samoa as the team prepares for the 2026 Rugby League World Cup, before joining Parramatta's pre-season training later this year.
The 26-year-old will move from the Sydney Roosters to the Eels in 2027 on a four-year contract reportedly worth more than $850,000 per season.
Leniu was mostly used off the bench during the 2026 NRL season, including in the Roosters' Grand Final victory over the Knights last Sunday.
Roosters coach Trent Robinson described Leniu's contribution in the final as 'momentum changing'.
The forward had permission to consider other options in pursuit of greater opportunities and leaves the Roosters with a year remaining on his contract.
Parramatta General Manager of Football Mark O'Neill said the signing would strengthen the club's future.
"We're thrilled to have Spencer join the Parramatta Eels and become part of what we're building," O'Neill said in an official club statement.
"He's one of the elite middle forwards in the game and has consistently performed on the biggest stages throughout his career.
"We know he'll make a tremendous contribution both on and off the field, and we can't wait to see him in Blue and Gold."
Leniu will play under Eels head coach Jason Ryles.