The former Gold Coast coach had led a list of candidates to replace Adam O'Brien, with Knights assistant Blake Green and St George Illawarra assistant Dean Young among the other options.
But the struggling club confirmed after a board meeting on Monday night that former Newcastle halfback Holbrook would return to NRL coaching next year, less than three years after being fired by the Titans.
The appointment, which includes a three-year deal, was made on the recommendation of a five-member subcommittee that included the club's last premiership-winning coach Michael Hagan.
"Following a thorough selection process, Justin was unanimously chosen as the best candidate to lead the club into a new era," the Knights said in a statement.
"We congratulate Justin on his appointment and look forward to his commencement as Knights head coach once his current commitments are fulfilled."
Since leaving the Titans, Holbrook has worked at the Sydney Roosters as assistant to head coach Trent Robinson.
The 49-year-old had been contracted with the Tricolours until the end of next season but Robinson previously indicated the club would permit him to leave for a head coaching job.
Holbrook had a 37.8 per cent win-rate across three seasons at Gold Coast, but led the perennial strugglers to their most recent finals berth in 2021.
He was controversially axed by the Titans in mid-2023 while the club was still in finals contention, to clear the way for Des Hasler's ultimately ill-fated stint.
Before that, Holbrook coached English side St Helens between 2017 and 2019, winning coach of the year in his final season and guiding the side to grand final glory.
At international level, Holbrook coached Papua New Guinea for three games in 2023.
He will be tasked with restoring hope to a Knights side that endured a dismal 2025 campaign, posting the worst attacking record in the league and conceding 66 points in their final game of the season.
O'Brien's departure was announced with two games to play in his sixth season at the helm.
On top of Holbrook's to-do list will be getting the best out of marquee recruit Dylan Brown, who arrives at the club in 2026 on record-breaking money.
Holbrook made his NRL debut at the Knights, where he was understudy to Andrew Johns, and has an understanding of what makes the footy-mad city of Newcastle  tick.
That connection, along with his noted abilities as an attack-minded coach with strong man management skills, have earned him another chance at coaching an NRL club that needs fresh ideas.
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