The 26-year-old made the revelation in his upcoming book, My Decade in the Premier League, extracts of which are being published by The Mirror.
"There was a mirror by Ronaldo's seat in the Old Trafford dressing room," said Rooney. "In the time I've been playing with Ronnie, the one thing I've noticed about him is that he can't walk past his reflection without admiring it, even if we're about to play a game of football.
"Every match, before the team goes out for the warm-up, he runs through the same routine. The kit goes on, the boots go on. Not long after, Ronnie turns to his reflection and stares, psyching himself up for the game.
"If there's one person with a bigger self-belief than Ronaldo, I haven't met him yet. He's not shy."
Ronaldo joined Real Madrid from United in the summer of 2009 for a record £80m.