Ronaldo headed home seven minutes into the second half and Casemiro added a second 10 minutes from time against a toothless PSG, who were without their injured talisman Neymar and had Marco Verratti sent off for a second booking in the 66th minute.
Ronaldo scored for the ninth Champions League game in a row, matching Ruud van Nistelrooy's record in the competition, before PSG striker Edinson Cavani equalised with 19 minutes left, only for Casemiro to give Real victory at the Parc des Princes.
Zinedine Zidane's side, who are 15 points adrift of arch-rivals Barcelona in La Liga but aiming for their third straight Champions League crown, were solid throughout as the home side rarely threatened and fully deserved their win.
Meanwhile Liverpool qualified for the last 8 for the first time since 2009 despite playing to a barren draw with Porto at Anfield, sailing through with a 5-0 aggregate scoreline having completed the job in the first leg.