The ex-England centre, 33, has played more than 300 games for the West Yorkshire club over two spells and led them to top spot for the first time in their history in 2017.
He led Castleford in their maiden Grand Final two years ago and to Wembley in 2014.
They will face newly-promoted Toronto – possibly with superstar Sonny Bill Williams featuring – at Wheldon Road on Sunday January 19 (2pm).
“It’s been an honour for me to represent the club in the role that I have done,” said Shenton, who debuted in 2004.
“I’m really excited for the Toronto game; they bring a lot of excitement and expectation with them, so I hope the fans will come down to support me and the rest of the boys, and see what Toronto are about.
“You could say a testimonial game is about that individual player but I don’t think so. It’s about the team and our preparations for the new year.
“We’re going to have a pretty strong side out for it and I know Daryl (Powell) wants to play a side that he’s more than likely going to go with for round one.
“It’ll be a smallish squad against Toronto and they don’t have a huge squad either so it’s pretty much going to be a Super League-standard game two weeks before the comp’ starts.”