Seymour, 29, leaves having failed to make a Super League appearance for the Tigers, and head coach Daryl Powell said the decision was mutual.
“We sat down and had a chat with Brett and came to a mutual decision that we would release him from his trial period,” said Powell.
“I’m hoping that he will get a club in the near future and we wish him all the best.”
Last year, during a battle with poor form and depression, he attempted to take his own life. After taking a combination of alcohol and sleeping pills, he drove towards the Humber Bridge with the intention of driving straight off, but instead he crashed into two parked cars.
He has since spent time in the Sporting Chance clinic set up by former England footballer Tony Adams.