The rider had the choice of last year’s Cheltenham hero or stablemate Denman, winner of the Hennessy and Lexus Chase for trainer Paul Nicholls.
Walsh described the decision as “huge” but added: “At the end of the day, how do you get off a Gold Cup winner?”
The rider’s decision to stick with Kauto Star leaves Sam Thomas free to resume his partnership with Denman and he will be aboard when the eight-year-old has has prep outing in Saturday’s Aon Chase.
Nicholls told the Racing Post: “Everyone knows it would be very difficult for Ruby not to ride Kauto Star again in this year’s Gold Cup.
“Paul Barber and Harry Findlay, joint-owners of Denman, had a chat yesterday and agreed it would be better to make a decision now regarding Denman which would mean Sam could ride the horse at Newbury on Saturday as well as Cheltenham.
“However, should anything happen to Kauto Star, God forbid, between now and Gold Cup day, Ruby, as my stable jockey, would switch to Denman.”