Melbourne will lose goal-kicking winger Nick Meaney to Perth in 2027 and Isaako is the ideal like-for-like replacement.
Isaako's partner Abby is from the Victorian capital and his manager Jim Banaghan said the move was the best for all parties.
"It's a win-win for the club, Jamayne and his family," Banaghan told AAP.
"He loves the Dolphins and is sad to be leaving but that is the reality of the salary cap these days. Hopefully one day he can return."
The loss of Isaako has implications for the Dolphins and their plans to re-sign several off-contract players on their books.
AAP has been told the club is in talks to sign off-contract winger Selwyn Cobbo and centre Jake Averillo.
They also have wingers Jack Bostock and Tevita Naufahu on the books until the end of 2027. LJ Nonu, yet to feature in the NRL but who plays centre and wing, is also signed for another two years.
Foundation winger Isaako has been a revelation for the Dolphins and has  played an incredible 76 matches consecutively. He has also played every minute of every game in a tribute to his reliability and immaculate preparation off the field.
On top of those impressive statistics, he was the NRL leading point scorer in both 2023 and 2025 after also winning the award in 2018 at Brisbane in his rookie season.
The 29-year-old, in 164 NRL games for the Broncos, Gold Coast and Dolphins, has scored 77 tries and landed 525 goals for a career tally of 1363 points.
His goalkicking will be a huge asset for the Storm. Meaney was absent with a back injury in the 28-24 loss to North Queensland on Saturday night and captain Harry Grant, not a recognised kicker, could only land two from five attempts and missed a couple of sitters.
Isaako will rectify that issue immediately when he arrives.
"Jamayne is a high-quality and experienced player who adds speed and versatility to our backline," Storm director of football Frank Ponissi said.
"With Jamayne's partner Abby having her parents based in Melbourne, this move suits both parties incredibly well, and we're confident he'll be an excellent addition to our club and make a strong contribution to our squad over the next few years."
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