They can re-sign the 23-year-old for next season, but have decided not to stand in the way of the young forward's opportunity to become a reality tV star.
In a statement on the club website, the Tigers said: "Castleford Tigers can confirm that the Club has agreed to release Jacques O’Neill from his contract to allow him to pursue another opportunity.
"The Tigers have the first option to sign O’Neill back next year should he wish to return to rugby league.
"Known for a fearless attitude on the field by Castleford supporters, the Cumbrian born 23-year-old is a proud product of the Tigers’ youth system having come through both our Scholarship and Academy programmes before making his Super League debut.
"Castleford Tigers would like to wish Jacques well with his new opportunity and thank him for his efforts as a Tiger so far.
"The Tigers would also like to say a big thank you to Russells who have sponsored Jacques O’Neill throughout his time at the club."
O'Neill will spend the summer on the ITV show after being confirmed as a contestant on the new series of Love Island.
He is putting his Rugby Leagure career on hold for now after being sidelined through injury for the last year after making his way up into the first team as a fiercely competitive hooker or loose forward.