The 28-year-old was released by Hull FC in April after a spell on compassionate leave.
Seymour was taken to hospital in March after drunkenly crashing his car in an apparent attempt to commit suicide.
The former Brisbane Broncos, Cronulla Sharks and New Zealand Warriors star said that he has worked through his problems off the field and is ready to get back to work.
"I can't thank [Castleford chief executive] Steve Gill and [coach] Daryl Powell enough for giving the opportunity to put things right," Sky Sports News quotes him as saying.
"The way it ended at Hull wasn't nice but I've got things sorted in my personal life which is the main thing.
"I've got to prove a point and that's out on the paddock. I'm healthy, raring to go and ready to be back playing and I'm happy to be here at Castleford."
Seymour joined Hull in 2012.