Already contracted until 2027, Iongi on Wednesday confirmed a new three-year extension that will keep him at the Eels until the end of 2030.
The move ensures he will be a key fixture of Jason Ryles' tenure at the club, after Parramatta released Clint Gutherson last summer to make way for Iongi at No.1.
The 22-year-old was then Parramatta's rookie of the year with eight tries in 2025, prompting the latest extension.
"This past year has been surreal, I came to Parramatta for an opportunity and now it's become home," Iongi said.
"I've built a great connection with Jason Ryles, the coaching staff, my teammates, and everyone at the club, and I'm grateful for the faith they've shown in me.
"We've grown so much as a team over the past year, and I can't wait to keep building our future together at this club."
Iongi's new deal makes him the longest-signed player on the Eels' roster, taking him past Mitch Moses and Tallyn da Silva, who are signed until 2029.
After debuting at Penrith, Iongi was one of the first signings the Eels made in 2024 after announcing Ryles would take charge of the club last season.
He grew in confidence as the year went on, busting more than four tackles per game and laying on 12 tries before debuting for Tonga in the Pacific Cup.
"Isaiah has developed into an important member of the team and has settled into the club and our environment very well since he arrived last year," Eels football boss Mark O'Neill said.
"With his positive mindset, strong work ethic and desire to improve, we see him as a long-term leader at the Eels.
"Jason, the coaches and the rest of the club are incredibly excited to see him continue to evolve over the coming years."
Elsewhere, Melbourne have lured second-rower Oryn Keeley away from the Dolphins, signing him on a two-year deal for 2027 and 2028.
Keeley could have replaced Eliesa Katoa in 2026, following his surgery for bleeding on the brain after three head knocks with Tonga, but the Storm have confirmed he will only arrive for 2027.
"Oryn is a talented young player who has already made a strong impression at first-grade level, demonstrating both skill and hard work," football boss Frank Ponissi said.
"We're excited to welcome Oryn and his family to Melbourne for the 2027 season."
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