England centre Shenton, who celebrates his 24th birthday today, is out of contract at The Jungle at the end of the season and is expected to join St Helens.
Tigers coach Terry Matterson said: "We would have liked to have kept Michael and made him a big offer to stay. However he has made a decision to move on which we respect and we are very grateful for his contribution in what will have been seven seasons with the club.
"Michael is also a real professional and I know that he'll be as keen as anyone over the remaining fixtures to help us win us a place in the top-eight.
Castleford have been given a boost with the news that highly promising young centre Joe Arundel has agreed a new deal with the club.
Arundel's current deal expires at the end of the season but he has now signed up for another two years.
He made his first appearance for the Tigers as a 16-year old substitute against Huddersfield in 2008 and has broken into the squad again this season, creating a big impression in his five appearances.
A leg injury has ruled him out of the Tigers recent games but he is hoping to be back in contention for this weekend's clash with the Crusaders.
Matterson said: "On the back of Richard Owen re-signing we're really pleased that Joe has also agreed to stay with the club for another two years. He has pushed his way into first grade this year and looked very comfortable. I'm very excited about what Joe can do for Cas in the future and we are hoping to see him available for selection for Sunday's game in Neath."
Arundel said: "I always wanted to stay with Cas so I'm pleased to have got this new deal sorted out. Breaking into the squad this season has been a plus and hopefully I can keep my place this year. And now I can also look for a big off season here and come back better still for 2011 and hopefully clinch a regular place in the team."