Ronaldo has been the star of Real's run to the showpiece at Estadio da Luz in Lisbon, scoring 16 goals in the competition this season.
However, the Portugal international has missed the last two games because of a muscle injury sustained in a 1-1 draw at Real Valladolid earlier this month, and pulled out of last weekend's 3-1 triumph over Espanyol in the warm-up.
The 29-year-old's participation in the final against city rivals Atletico had looked to be in question, but Ancelotti insists Ronaldo will definitely feature.
"I think the two (Ronaldo and winger Gareth Bale) are going to play," Ancelotti said.
Ancelotti does have doubts over whether striker Karim Benzema and defender Pepe will be available for the clash.
Benzema suffered a hamstring injury against Espanyol, while Pepe is still recovering from a calf problem.
Former Milan, Paris Saint-Germain and Chelsea boss Ancelotti expects Benzema to be fit in time, with Raphael Varane ready to step into the breach if Pepe is unavailable.
"I'm realistic at the moment, we've got four days to see how things go, and we'll make a decision (on Benzema and Pepe) at the end," Ancelotti added.
"Now I think Karim is fine. If we thought that there was a risk of injury, he wouldn't play and he wouldn't train.
"Varane can play and can replace Pepe."
Ancelotti is anticipating an even-matched final, adding that he will remain at the Bernabeu regardless of whether Real win Europe's premier club competition for a 10th time.
"There is no secrets about this match, the two teams know each other very well. Like all finals, it is very well balanced," he explained. "We know that we are playing against a very solid team. A difficult match for us.
"My future is very clear, regardless of what happens in this final. I am happy to continue here - with or without La Decima."
