Australian Liam Fulton, former Harlequins loose forward Joe Walsh are also leaving the Galpharm Stadium.
Reserve players Richard Lopag, Tom Hemingway and Chris Lawson will also be released by the club.
Huddersfield managing director Richard Thewlis said: "Jacko has been a tremendous player for our club.
He added: "He has an opportunity elsewhere that is the right one for him to accept."
The 30-year-old prop forward is expected to rejoin Castleford, while Fulton could return to Wests Tigers in the NRL.
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