The Broncos back became the third significant addition to the new franchise on Wednesday, Isaako signing a three-year deal that will begin after he finishes next season at Red Hill.
He is the first Bronco to switch allegiances to the cross-town rival.
Bennett handed Isaako a debut late in the 2017 season and thrown into fullback by Bennett, he flourished to claim the 2018 Dally M rookie of the year gong.
But Isaako struggled to build on that early form and has spent the past two seasons under pressure to retain his spot.
With a host of younger backline talent on the books at Red Hill, Isaako's future at the club appeared uncertain.
The Dolphins have shown an early degree of confidence in the 25-year-old though, club boss Terry Reader saying he's "pretty sure we already know who will be kicking goals for us in 2023".
"Jamayne is fast, elusive and a point-scoring machine, so we welcome him into the first-ever Dolphins squad," he said.
Isaako, who has played for both Tonga and New Zealand, joins Queensland State of Origin backrower Felise Kaufusi and Parramatta hooker Ray Stone as Dolphins signings.
"Jamayne has been in the Broncos system for a while now and made a wonderful contribution over many years," Broncos football boss Ben Ikin said.
"He remains contracted with the club until the end of the 2022 season and has spoken with us about making it a great year on and off the field.
"We wish Jamayne and his family all the best in the next part of their journey once we reach the end of the 2022 season."
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