Tyrell Sloan scored 10 tries in his final 11 NRL appearances, but will continue his career in England rather than pursue another opportunity in Australia.
The Daily Telegraph confirmed he intends to honour his agreement with Castleford Tigers and join the Super League club next season.
His management team has started arranging his relocation and working through visa requirements.
Sloan committed to Castleford while Ryan Carr was coach, and his relationship with Carr helped shape the decision.
Carr's mid-season departure, combined with Sloan's improved form, prompted speculation that he might stay in the NRL or remain with St George Illawarra.
The Dragons could not retain the outside back, who will join former teammates Damien Cook and Matt Feagai in England.
Despite the changed circumstances, Sloan has settled on the move, which offers him a chance to develop his career overseas.