The 21-year-old, who came through Huddersfield's youth system, has become Hull's seventh signing ahead of the 2013 campaign.
"I didn't know too much about the interest until last week," Walker told the club's official website. "[Head coach] Craig [Sandercock] had seen me play in a few games at the start of last year and told me that he was impressed with what he'd seen. We then had a chat about the possibility of a move and I'm just glad to be here now.
Sandercock added: "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. I watched him a few times last season and was very impressed with what I saw."
Walker made four Super League appearances for the Giants last season.