Leniu informed the Panthers on Monday morning this NRL season would be his last at the club, after making his debut as a hard-running forward in 2019.
The bad-luck story of their 2020 grand final appearance when sidelined by illness, Leniu came off the bench in both the 2021 and 2022 title wins.
Leniu will see out the season with Penrith, but is expected to sign with the Roosters in the coming days after initially also attracting interest from Canterbury.
The front-rower's signature is the second big fish signed by the Tri-colours from a rival club this month, after also locking in Newcastle flyer Dom Young last week.
At Penrith, the costs of their premiership success are starting to show.
Of the 27 players who ran out for the club in the first year of their dominance in 2020, a maximum of eight will begin next season at the Panthers.
Leniu's impending exit comes after Stephen Crichton gave Penrith management similar news last month.
The Panthers have a dominant junior system and last year won reserve grade and under-21s.
But they face a tough task replacing the likes of Apisai Koroisau and Viliame Kikau in NRL this year, and others going forward as the player drain continues.
"Although it's disappointing to see a Panthers junior sign with another NRL club, we understand Spencer is a premiership winner and international player, who has received external interest," Panthers Rugby League CEO Matt Cameron said.
"Panthers thanks Spencer for his efforts for the club and is looking forward to seeing him run out for the Panthers for the remainder of the 2023 NRL season."
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