Leniu finished his three-year stint at the Tricolours on a high after a stellar performance in their 19-18 win over Newcastle to secure the 2026 trophy.
The NSW Origin representative added the Roosters' premiership ring to the three he won with Penrith from 2021-2023.
Leniu was granted a release from the final year of his Roosters contract, which will give the club some salary cap reprieve
The hard-hitting forward's Eels contract is reportedly worth more than the $850,000 per year he was making with Easts, where he spent much of his time coming off the bench.
He is expected to play a more active role in blue and gold, leading the line for the Eels' forward pack.
The westward move will see the former Panther reunite with good friend Jarome Luai, but first the pair will both suit up for Toa Samoa at the Rugby League World Cup.
Eels general manager of football Mark O'Neill shared his delight at signing one of the game's "premier enforcers".
""We're thrilled to have Spencer join the Parramatta Eels and become part of what we're building at this club," O'Neill said in a statement.
"He's one of the elite middle forwards in the game and has consistently performed on the biggest stages throughout his career.
The exit of Leniu adds to the exodus of stars from the Roosters premiership line-up.
Angus Crichton and Mark Nawaqanitawase are rugby-bound, Daly Cherry-Evans is joining Hull FC, while Daniel Tupou retired this week after his 307-game career.
Connor Watson, who missed the title decider due to concussion, has signed for a season with St George Illawarra Dragons.
Roosters coach Trent Robinson said Leniu had been a strong contributor during his time at the club.
"Spencer has brought a lot to our group over the last three years, both through the way he plays and the energy and personality he brings every day," Robinson said in a statement on Thursday.
"He brings an intensity and competitiveness to everything he does, and he challenges the people around him to be better.
"To see him finish his time here as a premiership winner is something he should be really proud of, and something we'll always share together."
With his departure announced announced at the Roosters' awards night on Wednesday, Leniu said he would leave the club on a high.
""I'm proud to have worn the Roosters jersey and I'll always have a lot of love and respect for the club, the boys and everyone who has supported me during my time here," said Leniu.
"Winning a premiership with this group and sharing that with the boys, our families and everyone at the club is something I'll carry with me for the rest of my life."
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