The 22-year-old Ireland international joined Hull in 2018 but played most of his rugby on loan to Batley and Leigh and made only one Super League appearance in 2020.
Bienek is joining the Tigers on an initial 12-month deal but with an option to extend until the end of 2022.
“We tried to sign Lewis before he went to Hull,” said Tigers coach Daryl Powell. “He has loads of energy and enthusiasm and he’s a big kid with great leg speed – he’s pretty rapid.
“Lewis is a great prospect and I’m looking forward to getting him in and seeing what he can do.
“Front-row is an important position and I think Lewis will really add to us.
“We have a real air of youth that will stand the club in good stead for years to come which is what we’re looking for, but we’re also looking to win a competition next year as well.
“With that in mind, we still want to add another experienced prop for 2021 who can come in and make an immediate difference to our go-forward, and we’re in advanced talks with a Super League-based player who we’re confident can do that for us.”
Let's gooo!! 2021 🐯👊🏼 https://t.co/G2PakqvVjR
— Lewis Bienek (@lewis_bienek) November 24, 2020
Bienek said: “Daryl is one of the main reasons I wanted to sign with Cas.
“He’s a great coach and he gets a lot out of players that are coming through that are younger so it will be great to learn a lot from him and there are more experienced players there, too.
“The World Cup is coming up so I want to put my best foot forward to get into the Ireland squad again and show what we can do there again.
“There’s a lot to look forward to and a lot of challenges to really get stuck into.”
Listen to The Final Hooter podcast, featuring exclusive interviews, every week. SUBSCRIBE to Love Rugby League on Apple Podcasts.