Hull KR have signed former Leeds full-back from Championship side Bradford on a deal until the end of the season.
Walker, 23, has played five times for the Bulls this season and will remain available to play for them until called upon by the Robins.
He played for their city rivals Hull FC six times in 2022 after an injury-hit few seasons at Headingley.
Robins coach Willie Peters said: “We need some depth in our fullback position with Ethan (Ryan) breaking his jaw, so Jack (Walker) will give us much needed depth there.
“Jack has played at the highest domestic level and won a competition with Leeds. In my eyes, he’s a genuine Super League player and will fit in well with us at Hull KR. Also, it’s important that Jack plays every week so when he’s not in our squad, Jack will be available for Bradford.”
Bradford confirmed that Walker had a Super League clause in his contract, that was insisted upon when he joined them from Leeds ahead of the 2023 season.
Walker said: “I have loved every minute of it, I am grateful to be given the opportunity to play at Bradford even though I have only played a small amount of games I have enjoyed it.
“There is a great set of lads and people at the club, I am really thankful and I don’t think it will be last time I play at Bradford so I don’t think it’s a farewell.
“The game at the weekend I felt I really connected with the fans, to say I came from Leeds they were always warm to me, I felt really welcomed by everyone, fans, staff, players and hopefully it is not the last time I play at Bradford.
“Regardless of anything, my sole focus is on tonight and hopefully carry on from where we left against Keighley and have a great game and if it is my last, end it with a win.”
Bradford have also confirmed that Walker will be available to them when Hull KR do not need him, though they are actively in the market for a replacement to strengthen their squad.
Bulls CEO Jason Hirst said: “Once his Super League clause was activated and Jack informed us of his desire to join Hull Kingston Rovers, we neither could nor indeed morally wanted to stand in Jack’s way.
“Rugby League is a short career and while protecting the club’s best interests, we would never stand in a player’s way in terms of career progression and the opportunity to play at the highest domestic level.
“The fact that he is joining a top end Super League club is testament to Jack and his professionalism and determination to get himself fit and back on the field of play.”
Hull KR’s first choice full-back Lachlan Coote announced recently his intention to retire at the end of the season, having suffered numerous concussions this campaign.
Will Dagger was allowed to depart to join Wakefield in a swap deal that involved Corey Hall moving to Craven Park, while stand-in Ethan Ryan suffered an injury in last week’s defeat to Wigan.