Mason, 30, had a short spell with the Tigers in 2006 before moving to Huddersfield, where he spent the last six years before acrimoniously leaving the Giants last month, despite having three years left on his contract.
The former Wakefield and St Helens forward, who has also played in the NRL with Melbourne Storm, had fallen down the pecking order at the Galpharm Stadium, with Huddersfield claiming "the matter is in the hands of solicitors".
He has now become Castleford's eighth signing of the close season as they attempt to better their woeful 13th place finish in the last campaign.
"I was in search of an experienced forward who could offer a presence both in training and on the field and Keith fits that bill," said coach Ian Millward, who worked with Mason when the two were at St Helens.
"I know him to have one of the best work ethics in the game; he commands respect from his peers and will no doubt have a positive impact on the team.
"I am confident that Keith still has some of his best years ahead of him which should also create some healthy competition between the forwards as we gear up for next season. He starts with immediate effect and will join the squad in training tomorrow."
Mason joins Lee Gilmour, Michael Shenton, Jake Webster, Jamie Ellis, Jordan Tansey, Justin Carney and Weller Hauraki as Castleford's other recruits.
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