25-year-old Isaako will join the new NRL club on a three-year deal starting in 2023.
"Jamayne is fast, elusive and a point-scoring machine, so we welcome him into the first-ever Dolphins squad,” said Dolphins’ CEO chief executive Terry Reader in a statement.
"Jamayne is a great pick up for the Dolphins for our first year – it means we are pretty sure we already know who will be kicking goals for us in 2023."
Isaako was the NRL's Dally M Rookie of the Year in 2018 and has also played five Tests for New Zealand.
Isaako remains contracted to the Broncos for 2022.
"Jamayne has been in the Broncos system for a while now and made a wonderful contribution over many years," Broncos head of football Ben Ikin said on the club website.
"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."