The 28-year-old Portugal international played for City's Premier League rivals Manchester United for six years between 2003 and 2009.
"To play for City not United? I'm not going to do that," he told The Sun. "Why? Because my heart's in Manchester United, that's why!"
Ronaldo added that he is "happy" at Madrid but admits that in the future anything is possible.
"I am a player of Real Madrid and I'm happy here, my family is happy and everything is going well for me here," he said when asked about returning to United.
"The future? I don't know — only God knows the future. I can't say I'm never going back to Man United — but I can say I'm good here and I want to be here.
"Let's leave it at that and see what God gives me in the future — but I repeat I am happy here."
Ronaldo joined Madrid from United in the summer of 2009 for £80m.