The Australian linked up with Tigers in the off-season after being released by Hull mid-way through 2013 following a battle with depression.
Seymour was unable to force his way into Tigers’ Super League side and is now looking for another club.
Tigers coach Daryl Powell confirmed: “We sat down and had a chat with Brett and pretty much came to a mutual decision that we would release him from his trial period, which he was still on.
“Obviously our season has started pretty well and Liam Finn, Marc Sneyd and Jamie Elllis - who’s in front of him as well - left him fourth in line.
“We wanted to give him the best opportunity to pick up a club, because the way rugby league is at the moment he just can’t play. That is disappointing, but it’s the way it is.”
Powell added: “I have to say he’s been fantastic while he’s been here. He has fitted in well and got on well with the players and he has worked hard, really hard.
“I am hoping he will get a club in the near future and we wish him all the best. I think he will be full of praise for the players and the work we’ve done with him to enable him to have an opportunity.
“It is disappointing for us and Brett that it hasn’t worked, but that’s life. Sometimes it rolls that way and our excellent start for the season has made it even more difficult for him to get in the team.
“The last thing he wants to be is a professional trainer, so I think it is the right decision for us and Brett that he moves on and hopefully gets a club over here as soon as possible.”
Powell also confirmed centre Jake Webster, who has not played for almost a year due to a serious knee injury, will play for dual-registration partner club York City Knights away to Hunslet Hawks on Sunday.