Justin Holbrook will leave Super League leaders St Helens at the end of the season after being named boss of Australian National Rugby League side Gold Coast Titans from 2020.
Holbrook, 43, will be looking to guide Saints to an English treble before moving, with his side top of Super League and in the Challenge Cup final.
His move back to the NRL comes after two and a half years with Saints.
"It's been an extremely tough decision to leave such a great club," he said.
"Since I arrived, everyone at the club and the fans have been brilliant to myself and my family and we have loved our time here," added Holbrook, who has signed a two-year contract with the Titans.
"(Chairman) Eamonn McManus, (chief executive) Mike Rush and the board wanted me to stay and did everything they could, which I greatly appreciated, but an opportunity to return home to the NRL is what I feel I need to do.
"It is very difficult as the only time I would get an opportunity like that is when we are winning and doing well as a club, which makes the decision even harder for me."
Saints have said they will not make any comment about Holbrook's replacement until after the Challenge Cup final on 24 August.
Saints to the summit under Holbrook
Holbrook was named St Helens head coach in May 2017, arriving as a full-time replacement for Keiron Cunningham, who had left a month earlier with the side struggling in seventh spot.
The Australian left his job as Sydney Roosters assistant coach to take up his first role as head coach, having previously worked on the backroom staff at Canterbury, St George Illawarra and Parramatta.
In his first full season in charge, Holbrook led Saints to Super League's summit and collected the League Leaders' Shield.
However, that 2018 campaign ended in disappointment as they failed to get beyond the semi-final stage of both the Super League play-offs and Challenge Cup.
With the League Leaders' Shield again likely to be theirs, as Saints are 10 points clear at the top of the table with six regular-season matches remaining, there are great expectations on Holbrook's side to deliver on all fronts.
They secured their place in the Challenge Cup final against Warrington on 24 August after battling past part-time Championship club Halifax on Saturday.
Reaching Wembley for the first time in 11 years for that decider ends a drought for the club, but going on to win that and giving Holbrook a glorious send-off in the Super League final at Old Trafford on 12 October now becomes a major focus.
Holbrook will take over a struggling Gold Coast side that sacked Garth Brennan earlier in July with the club bottom of the NRL.
In his Titans squad will be England centre Kallum Watkins, following his mid-season move from Leeds.
Holbrook 'instilled a belief' at Saints
Analysis: BBC Sport's Matt Newsum
St Helens and Holbrook have both benefitted from the arrangement which saw the Australian come to Super League, and it will be interesting to see just how they go once they are apart after this season.
Saints were a side struggling with their identity when Holbrook arrived. He instilled a belief, and most importantly restored the DNA of the club with a swashbuckling style of play. Forwards Alex Walmsley and Luke Thompson have thrived, becoming international players in a ferocious forward pack.
The signings of 2018 Man of Steel Ben Barba, and then his replacement Lachlan Coote have added to a devastating back division which has brought the best in turn from Great Britain hopefuls Tommy Makinson, Regan Grace and Jonny Lomax.
One question mark over Holbrook, for all the rapid development and creation of a winning core, is the lack of silverware. Before he leaves there is the opportunity to win the treble, much as there was in 2018, but Saints lost their nerve in two semi-finals and failed to fulfil their sparkling promise.
They are already over that hurdle in 2019 with a Challenge Cup final appearance beckoning, while a 10-point lead in the Super League table highlights their superiority.