Maher has spent the last five years at Wheldon Road making nine appearances this season but also been on loan with Halifax, Oxford, Batley and Keighley.
“I am really excited to join Rovers." he said.
"I have played against the club a few times and have experienced the crowds before. I am very excited to be working under Tony Smith here.
“I had a really good few years at Castleford, but it was definitely time to move on and go somewhere that I can reinvent myself.
“I am looking to progress under Tony’s stewardship. I like the way that Tony goes about his business. He is more about the person, than just the ‘rugby player’, which I like.
“He took a lot of interest in me personally rather than just talking rugby-wise.
"I am looking forward to developing that strong relationship on and off the field, we got on really well.”
KR needed to reinforce their front-row options after the release of Lee Jewitt and early retirement of Nick Scruton mid-season.
Maher added: “I have worked with Weller Hauraki when he was at Castleford and I am excited to be with him again.
"There are lot of good senior props here and I just want to improve and enjoy myself.
“I want to be a Super League regular and ensure I am fit and available for selection as much as possible."
Head coach Tony Smith commented: “Will is now entering an age of maturity for forwards.
"He is an ambitious young man who wants, and seeks, a change and an opportunity to try and establish himself as a more regular first-team player.
“I think he has done a good job for Castleford in the time that he has had, but he has a strong desire now to kick on and progress his career here.
“He knows there are some areas that need to be developed and he is quite open and honest about that with me. I am happy to help him do that. He has great framework to build and work on and a great desire to be successful.
“Will understands that he is not the finished product by a long shot, and he will learn a lot from the experienced players here which bodes well for his development across the next two years.”