The 35-year-old is in top form, but the Knights were not in a position to offer him a new contract for 2027.
Instead, he will return to the Broncos club he last played for in 2012 when he had his contract torn up after a string of disciplinary breaches.
That early glitch in his career turned out to be a blessing in disguise as he joined the Novocastrians under mentor Wayne Bennett and then played under him again at South Sydney before returning to Newcastle.
Gagai, who has played 328 NRL games, became one of Queensland's most reliable finishers in 23 State of Origin games and represented Australia in seven Tests
Gagai will join a Broncos side that have already lost outside back Jesse Arthars to Catalans for next year and that is expected to be without Test centre Gehamat Shibasaki in 2027.
Brisbane recruitment boss Simon Scanlan said Gagai was a wonderful acquisition and would be a fine mentor to the club's young outside backs.
"The Broncos are very excited to see Dane making a return to Red Hill in 2027," he said.
"His form this season has been outstanding and he has undoubtedly been one of the form centres this year.
"As we continue to develop our young outside backs, having experienced professionals around them is incredibly important. Dane's leadership, professionalism and proven performance at the highest level make him a valuable addition to our club."
Meanwhile in other signing news, Dolphins utility Jake Averillo has inked a four-year deal with Wests Tigers from 2027.
Averillo has been a valuable player for the Dolphins in a variety of outside back positions and in the halves, but salary cap constraints meant the club could not match the lucrative offer and length of term provided by the Tigers.
Gold Coast forward Klese Haas is set to stay at the Titans until the end of 2029 after extending his stay for two years beyond 2027.
The move will put a delay on his hopes of one day lining up next to older brother Payne, who will join South Sydney next season from Brisbane.
"Living on the Gold Coast basically my whole life, it means the world to me and my family to lock in a further two years here," Haas said.
"I'd be lying if I said the temptation wasn't there to play together with Payne, but at the end of the day, this club has given me so much and I want to repay that faith here at the Titans.
"The upwards trajectory this club is going in is exciting to see and I can't wait to keep being part of it."
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