Isaako capped a remarkable season with the one-year extension after he was the NRL's leading point scorer and try scorer with 244 points and 24 tries.
His stellar form earned him a recall to the New Zealand Test squad, after a four-year absence, for the upcoming Pacific Cup series against Samoa and Australia.
The Dolphins player of the year was already on the club's books until the end of 2025 and club CEO Terry Reader said his new upgraded deal was a reward for a season where he played all 24 games at the highest level.
"Jamayne could simply have not played any better in 2023," Reader said.
"To become the first person in 33 years to be the NRL's top try scorer and point scorer in the same year was an outstanding effort.
"And to do that in a team that finished outside of the top eight makes it an even greater achievement. When Mal Meninga did it (for Canberra) in 1990, his team won the grand final.
"The dedication and drive Jamayne has showed this year to be the best he can has turned him into an elite player in the competition.
"So extending his contract to ensure he is a Dolphin for at least another three years was an easy decision for the club to make."
Isaako joined the Dolphins from Gold Coast when his career was stalled.
Dolphins coach Wayne Bennett gave Isaako an ultimatum in pre-season that drove him on.
"Wayne told me right before the season was supposed to start that he was going to start me in Q Cup and as much as it broke my heart I was adamant that I was going to prove to him that I wanted to play in this team," Isaako said.
"I got the opportunity to start round one and I knew it was an opportunity I couldn't look back on.
"I took it so serious at the start of the season that this year was going to be different, and it was different."
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