The Portuguese dispelled any doubts over his form by slotting home the opener from Luka Modric’s wonderful pass before doubling his tally from the penalty spot after Gareth Bale had been felled inside the area.
Bale crossed for Ronaldo to complete his 32nd hat-trick for Madrid and the World Player of
the Year then turned provider himself for Karim Benzema to tap home a fourth before half-time.
Victory moves Madrid to the top of the table, one point ahead of Atletico Madrid and Barcelona who face off later on Saturday.
Ronaldo’s five-goal haul takes him ahead of Madrid legends Raul and Alfredo di Stefano to become the club’s all-time leading scorer in La Liga with 330 goals.
“Someone who scores so many goals has their place in history forever like Raul or Di Stefano,” said Madrid boss Rafael Benitez.
“I think we are speaking of three historic players and I hope Cristiano continues adding to his tally for his good and the good of the team.”
Los Blancos were in action for the first time since their botched deal to sign Manchester United goalkeeper David de Gea collapsed due to the paperwork between the clubs being completed minutes after the Spanish transfer deadline.
However, Keylor Navas proved he can be a worthy successor to Iker Casillas with a fine save from Felipe Caicedo to prevent Espanyol getting off to a flying start.
The result was never in doubt once the visitors took the lead seven minutes in, though, as Ronaldo controlled Modric’s brilliant through ball before placing the ball inside the far post.
Bale was playing once more in a central role behind Benzema in attack and produced his best performance in a Madrid shirt for some time as his driving run into the box was cut short by Alvaro Gonzalez.
Ronaldo converted the resulting penalty confidently and was the beneficiary of another Bale assist moments later to complete his hat-trick after just 20 minutes.
Another surging run from Bale helped set up the fourth as he fed Ronaldo and the former Manchester United man squared for Benzema to apply a simple finish.
Benzema could have even added a fifth before half-time as his driven effort was brilliantly tipped onto the post by Espanyol ‘keeper Pau Lopez.
Bale was the creator once more when the fifth did arrive just after the hour mark as he unselfishly teed up Ronaldo to roll home his fourth of the afternoon.
And Ronaldo matched his own personal record of five goals in a game when Madrid smashed Granada 9-1 last season by thumping home Lucas Vazquez’s pass 10 minutes from time.