Harrison, on contract until 2027, was days away from being able to negotiate with other clubs from November 1.
The PNG Chiefs were in the hunt to table on offer but now the Titans have taken that option out of the equation, with the rookie committing to the club until the end of 2029.
Harrison impressed in his 11 NRL matches this year after coming through the club's Future Titans program and attending local league powerhouse Palm Beach Currumbin State High School.
Gold Coast now have New Zealand fullback Keano Kini on the books until the end of 2030 and five-eighth Jayden Campbell wrapped up until 2031 to provide stability in the key positions.
The Titans have previously shown interest in signing Melbourne hooker Harry Grant and are preparing to table the Australia hooker a lucrative offer once the becomes a free agent on November 1.
Gold Coast, who finished 16th in 2026 for a second consecutive year, gave rising forward Cooper Bai permission to negotiate with the Chiefs after the 19-year-old made it clear he did not intend to re-sign.
It was imperative that the Titans did not lose Harrison and coach Josh Hannay was celebrating a key win on Thursday after the young gun put pen to paper.
"Since I joined the club, Zane has consistently spoken about three things ... his teammates, his love of the Titans and his desire to bring success to his home town," Hannay said.
"He's one of the most impressive young men I've worked with and demonstrates day-in, day-out the qualities we want as a Gold Coast Titan.
"He's tough to go with his talent and his temperament is what sets him apart as a young playmaker."
Hannay admitted there had been "plenty of speculation about Zane's contract situation over the past month", but added that through the negotiation process he was convinced Harrison wanted to be part of a future renaissance at the club.
Harrison grew up in Tweed Heads supporting the Titans.
"I'm really excited about what Josh is building and the direction this team is heading. I want to be part of that," he said.
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