Ben Barba is to return to the NRL at the end of the season, having agreed a deal with North Queensland.
The 29-year-old, who spent 16 months in Super League, confirmed one of rugby league's worst-kept secrets at Saints' end-of-season awards.
St Helens brought Barba back into rugby league from a brief spell with French rugby union club Toulon, and have agreed an undisclosed transfer fee with the Cowboys and will announce a replacement shortly.
That is expected to be the Cowboys' Scotland international full-back Lachlan Coote, who has been released by the NRL club.
"Playing in England and St Helens in particular has been a wonderful experience for myself and my family," said Barba.
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"I want to finish the season off playing my best football for the club and fans and hopefully capping off a successful season."
St Helens chairman Eamonn McManus said: "It's sad to see Benny depart to North Queensland Cowboys in the NRL next season. We have respected his wishes and have obtained an acceptable transfer fee. We will also be announcing a new signing from the NRL shortly.
"Benny made a hugely positive impact upon his arrival at St Helens and both the club and the Super League competition have each benefited accordingly. It's now his intention to finish the season as he started it."
Barba signed for Saints last May after rejecting an offer from Warrington but had to wait until August to make his debut after losing an appeal against a 12-match ban imposed by the NRL after he tested positive for cocaine while at Cronulla.
Saints coach Justin Holbrook said: "Benny has been fantastic for us this season. From round one to 16 he was brilliant, but he picked up some niggling injuries which has hampered him over the last 10 or so weeks.
"He is now over those and wants to finish the year strong. We would have loved to have kept Ben for next season but he wants to return home and to the NRL. It is hard to stop that from happening but it hasn't affected himself or the team with this decision."
