The Roosters announced that the 26-year-old had been granted an immediate release from the final year of his contract, allowing him to join Parramatta.
From the premiership line-up Angus Crichton and Mark Nawaqanitawase are rugby-bound, Daly Cherry-Evans is joining Hull FC, while Daniel Tupou has 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.
Leniu finishes his three-year stint at the Tricolours on a high after a stellar performance in their 19-18 win over Newcastle secure the 2026 trophy.
"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," Roosters coach Trent 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.
"Spenny has a strong moral compass. He knows what he stands for, he cares deeply about his teammates and the people around him, and he's never afraid to speak up for what he believes in.
"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."
The title adds to the three consecutive premierships that the hard-hitting forward won with Penrith from 2021-2023.
With his exit 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."
The prop came across from the Panther's on a monster $850,000 per year contract, although has spent much of his Tricolours career coming off the bench.
Granting a release will give the club an opportunity for some salary cap reprieve.
Leniu's move to the Eels has not yet been made official, but the signing been one of the season's worst-kept secrets with speculation linking him to the club for months.
A shift to blue and gold 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.
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