Tigers say they have the option to sign Hardaker on a permanent contract in November next year and a transfer fee has already been agreed between the clubs.
Hardaker was transfer-listed by Rhinos last summer before ending the season on loan at Australian club Penrith Panthers, but is contracted to Leeds until the end of 2019.
The move reunites him with Tigers coach Daryl Powell, who gave Hardaker his professional debut at Featherstone Rovers in 2010.
Huddersfield Giants were also chasing Hardaker - who was England’s full-back in last year’s Test series win over New Zealand - but the Pontefract-born player preferred a move to Tigers.
Hardaker has scored 67 tries and 45 goals in 155 appearances for Rhinos.
Powell said: “I am delighted Zak has decided to sign for the Tigers. Two years ago he was the Man of Steel and there is no doubt he is an outstanding rugby league player.
“His flexibility in playing a number of positions in the back line will clearly enhance what is already a very talented squad and add quality and competition.
“I have worked with Zak in the past and look forward to seeing him enhance his own reputation as a rugby league player and get back into the England team as a Castleford Tigers player.”
Rhinos chief executive Gary Hetherington said: “I would like thank [Tigers chief executive] Steve Gill and the Castleford Tigers for how they worked with us on this arrangement.
“Zak leaves us this season with our best wishes.”