Lillycrop, 22, has previous Super League experience with Wakefield after coming through the ranks at Bradford.
He has played international rugby league with the England Academy and joins the Eagles on an initial one year deal.
The prop forward could feature in Sunday’s squad which will see him take on his old club the Rams, who visit Owlerton Stadium (23rd March, kick off 2pm).
Speaking of the signing, Sheffield head coach Mark Aston said, “It’s great to announce that Tom is joining the club.
“For me, he was one of the stand-out players at Dewsbury last year and we hope he will make a big impact here.
“He’s the type of player we want here, young, talented and lots of potential. We believe we can develop him further and help with his progression as a rugby league player.”
He will provide firepower and depth to the Eagles squad; something Aston feels is needed.
He added, “Everyone is aware that the pitch is a bit smaller at Owlerton than we’ve been used to. We are adapting to it, and in time we will turn the stadium into a fortress.
“We need more strength in depth, especially across our front row and Tom helps provide that.”
Lillycrop said, “I’m excited to be joining the Eagles. I’ve heard a lot of great things about the club and I can’t wait to get out there and begin playing.”