The New Zealand-born prop started out at Hull KR, playing a handful of games for them, but has also featured with Wyong Roos, Auckland’s Point Chevalier Pirates and Whitehaven.
He is the son of former Hull FC and Featherstone Rovers player Brad Hepi who also represented Castleford in 1999 and again two years later.
Despite being born in Auckland, Hepi is considered to be federation trained and, therefore, does not count on the club’s overseas quota.
He can also play back-row and Tigers - who have signed him as a replacement for Wigan Warriors-bound Mitch Clark - have a further one-year option in their favour for 2021.
Hepi, 25, has been one of Toulouse’s top performers and, on joining Castleford, he said: “I haven’t really given it much thought yet.
“I’ve been focused on getting up to Super League with Toulouse.
“I’ve been here four years and it’s been the goal for all that time so hopefully I can earn promotion here before coming across to Cas and having a shot next year.
“I feel like I’ve gone the long way round in my career; I’ve not taken any shortcuts.
“Looking back, I wasn’t ready to be a Super League player the last time with Hull KR.
“It did leave a bitter taste in my mouth when I was younger and I’ve been trying to get that back ever since.”
Hepi added: “I used to be a bit loose out on the field but coming to Toulouse the coach here has coached that out of my game.
“I was a bit raw but full credit goes to him for making me the player I am today and I feel ready for this next step.
“I’m really excited about coming to Cas and I just want to cement a spot in the side, play week in, week out with the boys and focus on becoming a better player.
“Being realistic, I know that I’ll be starting from the bottom and working my way up but I’m hungry as can be to prove myself.”
Castleford head coach Daryl Powell said: “Tyla is a quality young prop who I’m absolutely confident the fans will love.
“His dad played for the club and he has the same attitude as him.
“He will take Mitch Clark’s place in the squad and will fill it with commitment, dedication and desire.
“He is a tough defender and carries with real impact. I’m really looking forward to seeing him play in our great colours next season.”