The Halifax-born Wardle, who turns 26 in September, followed his old Huddersfield coach Nathan Brown to the NRL at the end of last season, joining Newcastle Knights on a two-year contract in a deal that saw full-back Jake Mamo move to the Giants.
Wardle has made a useful contribution in his debut season, with four tries in 13 appearances for the struggling Knights, but is cutting short his stay for personal reasons.
Newcastle chief executive Matt Gidley, the former St Helens centre, confirmed on Wednesday that the Scotland international, who is in coach Steve McCormack's initial World Cup squad, will be released at the end of the season.
"It is disappointing to lose Joe but working through this matter together has delivered a positive outcome for the club and for Joe and his family," Gidley said.
"For now, we look forward to having Joe with us for the remainder of this season."
Wardle, who began his career with Bradford and scored 58 tries in 125 Super League appearances for the Giants from 2011-16, said: "I would just like to thank Newcastle for helping me through this tough time and understanding my circumstances.
"It was always going to be a tough task moving to the opposite side of the world with a young family and there was a chance it wouldn't work out, which is unfortunately the case.
"The future of the Knights is definitely bright and I wish them all the best going forward."
It is thought Warrington and St Helens were keen to sign Wardle but that he has agreed a three-year deal with Castleford, who go into Thursday's Super 8s home game against Wakefield needing just a point to clinch the League Leaders' Shield.
They're 1/6 with Sky Bet, who make them 16-point favourites on the handicap, and 11/10 to win the Grand Final.
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