Monaghan played 118 games for Wire during his spell there, scoring 114 tries.
“I’m really excited. This is a team that are challenging for trophies and top fours,” said the former Australian international.
“Cas are a very good coached side and they are a very good bunch of blokes. I know a couple of the guys but I’m looking forward to preseason and getting to know the rest of them.
“I’m looking forward to being on the other side of the fans, it’s a great place down here at the Jungle and a very hard place for teams to come and get a win.
“When I told my dad I’d signed here he was happy because whenever he comes over the Jungle is always his favourite ground to come to.”
Tigers coach, Daryl Powell, is pleased to have boosted his backline with a proven try-scorer.
“Joel Monaghan is a high quality and experienced winger to add to our back line,” he said.
“He is a proven try scorer with good leadership skills and i expect him to help drive our standards in 2016.”