The 21-year-old forward, whose twin brother Jonathan plays for Castleford, has signed a three-year deal with the Robins after the clubs agreed an undisclosed transfer fee.
Rovers coach Craig Sandercock said: "Adam fits the profile that we're looking for in that he's young, he's English and he's very keen to learn and improve as a player."
He added: "I watched him a few times last season and was very impressed with what I saw.
"He has a lot of potential and, if he works hard and maintains the right attitude, then he has a great chance of establishing himself as a Super League player."
Walker, a product of the Giants academy system, made four Super League appearances last season when he also played on a dual registration for Championship club Swinton.
He said: "Craig has told me how he sees me fitting into the squad and what he expects from me so I'm just looking forward to getting stuck into pre-season.
"It was tough leaving Huddersfield because I've been there a long time and know everyone but the staff and lads here at Hull KR have made me very welcome."
- Sports & Recreation
- Craig Sandercock