Spencer Leniu ran for 92 metres in 24 minutes during the grand final and is now set for a more prominent role at Parramatta.
The Eels confirmed the four-time premiership-winning prop had signed a four-year deal, just hours after the Roosters announced his immediate departure.
Leniu was contracted to the Roosters until the end of next season but received an immediate release.
Parramatta general manager of football Mark O'Neill said: "We're thrilled to have Spencer join the Parramatta Eels and become part of what we're building at this Club. He's one of the elite middle forwards in the game and has consistently performed on the biggest stages throughout his career,"
The move reunites Leniu with former teammate Jarome Luai and gives the Eels added force up front.
Earlier in the year, CODE Sports reporter Brent Read said on NRL 360 that there were concerns about whether the 26-year-old was providing value for money.
Read said: "He was brought in to become a starting front-rower and he hasn't become a starting front-rower,"
He added: "He's paid to be a starting front-rower (but) he's being playing 20-30 minutes a game basically. You can't pay guys that amount of money."
Leniu left the Panthers for the Roosters, whom he called the "pinnacle of rugby league" in 2023.