Saints coach Justin Holbrook will leave the club at the end of the season to take up a contract with NRL side, the Gold Coast Titans.
St Helens chairman Eamonn McManus has vowed to bring in a "top class replacement" for 2020.
He said: “From a St.Helens perspective, we are clearly disappointed that Justin has ultimately decided not to enter into a new contract with us. We did everything practical to persuade him to stay, but we must respect his decision to join the Gold Coast Titans and to return to Australia with his young family. We wish him every success and more in his career and life ahead.
“We will leave the real thank yous and goodbyes until the end of the season. There are jobs to do and titles to be won, before then at Wembley and at Old Trafford and all focus must and will be on those. Justin and the entire club are now interested in that and that alone.
“We will immediately commence the search for a top class replacement and are confident that the Saints will successfully appoint one. We have a fantastic young squad which we fully intend to improve further in the seasons ahead.”
Holbrook, who has steered his side to three wins over Wigan this season, said: “It has been an extremely tough decision to leave such a great club in St Helens. Since I arrived here in 2017, everyone at the club and the fans have been brilliant to myself and my family and we have loved our time here."