Walker, 22, joined from Huddersfield last winter on a three-year deal, making 27 appearances and winning the club’s Young Player of the Year award.
He said: “I couldn’t be happier. With me having two years left on my original deal I wasn’t even thinking about an extension really. My only focus has been to try and play well every week and keep improving.
“The club came to me about extending my contract and it was an easy decision for me to make as I’m loving it at Hull KR. We had a good season in 2013 and have made some impressive signings for next season so hopefully we can improve again.”
Rovers Head Coach Craig Sandercock said: “Adam has really improved this year and his game 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.”