Garratt, 26, joins on a two-year deal having impressed in the Championship for a number of seasons.
Standing at six foot fives inches, it is hoped he has all the attributes to step up to Super League.
“We’ve been really impressed with what form he’s in," said Rovers coach Tony Smith.
"He’s really stood up to a lot of rigours of the Championship and stood out.
“He’s a big strong, ambitious young man with some maturity. Probably in his prime for a prop.
“(Tom) feels he’s ready for the full-time environment and wants to throw everything into becoming a full-time player and fulfilling his ambitions.
“He’s the right mould of what we’re looking for, to give us more depth and more go forward.
“I can’t see any reason why he won’t be a success at Super League level. We’ve identified that in his ability but also in his ambition and his drive.
“I’m really excited about the addition and what Tom’s going to bring to us.”
Meanwhile, young half-back Mikey Lewis has been allowed to rejoin York City Knights on a two-week loan - and goes straight into contention for their 1895 Cup Final appearance against Featherstone Rovers on Saturday.
However, that game is under threat due to a Covid outbreak at Featherstone.