Luke Thompson will leave St Helens for NRL side Canterbury Bulldogs with immediate effect.
The 25-year-old had been due to move to Australia for the 2021 season but the transfer has been brought forward.
Thompson has been with Saints since the age of 11, and featured for Great Britain on a tour of the southern hemisphere last year.
The prop won the Harry Sunderland man-of-the-match award in the 2019 Super League Grand Final, where Saints beat Salford, and was voted into the competition's Dream Team line-up.
Thompson told his new club's website: "Getting the chance to join the Bulldogs and be a part of a club with so much history and with such passionate fans is very exciting.
"Having spoken to many people at the club and listened to their plans for the future, I know that they are building for success and I'm looking forward to being part of that."