The Portuguese joined Madrid from United in the summer of 2009 and has scored a stunning 271 goals in 258 games for the Spanish club.
It has been widely reported that the attacker wants an emotional return to United this summer, but Ancelotti has said that the 29-year-old "will end his career at Real Madrid".
"Everyone can be changed and replaced but Ronaldo is the best player in the world. If he left Madrid it would be a huge loss," Ancelotti told reporters at a press conference.
"But that won't happen because he will end his career in Madrid. I am 100% sure Ronaldo won't be at Manchester United next season. 100%. I can't be wrong because he is happy in Madrid. Everyone wants to be here.
"The club is delighted to have the best player in the world. At the moment the relationship between club and player is perfect. So I don't think that is going to change. Ronaldo will end his career at Real Madrid."
Ronaldo has netted 19 times in 12 games this season.