The Portuguese had been seen as a doubt for the La Liga clash after picking up a calf problem during the Club World Cup final against Gremio in Abu Dhabi on Saturday night.
Zidane, however, has declared the 32-year-old "ready to go" for the match against Barcelona at the Bernabeu.
"Cristiano is 100% fit and ready to go," Zidane told reporters. "He has trained very hard and done everything right in order to be ready for this fixture."
Real Madrid will enter Saturday's clash with Barcelona three positions and 11 points behind their rivals in the table.
Ronaldo has only scored four times in 11 league appearances for Los Blancos during the 2017-18 campaign, although he has 16 in 21 fixtures in all competitions.