The 22-year-old, who was named as the club's young player of the year, has decided to prolong his stay at New Craven Park by two years, running until the end of 2017.
"I couldn't be happier," Walker told the club's official website. "With me having two years left on my original deal, I wasn't even thinking about an extension really but it was an easy decision for me to make as I'm loving it at Hull KR."
Coach Craig Sandercock added: "Adam has gone from strength to strength. He trains hard, listens, is willing to learn and has a great attitude.
"Adam will only get better in the years to come and it is exciting to know that he has committed to Hull KR for the next four seasons at least."
Walker, who has racked up 27 appearances for the club, joined the Robins from Huddersfield Giants last year.