The Portuguese hit a 13-minute hat-trick inside a rip-roaring first-half display, with Karim Benzema also on the scoresheet.
Ronaldo, criticised for his lack of goals as of late, then struck two more after the break to stun the hosts and make it back-to-back big wins for the Blancos, who beat Real Betis 5-0 last time out.
Here, Sports Mole analyses how the game was won in Catalunya.
On target: 3
On target: 8
Was the result fair?
The result? Undoubtedly, but the scoreline might have perhaps flattered Real Madrid. Bizarrely, it was Espanyol who started better but they swiftly paid the penalty for their coach's tactical mistakes. Even still, they had more than enough opportunities to restore an element of respectability to the scoreline, as the stats illustrate.
If Espanyol were like lambs to the slaughter this afternoon then coach Sergio Gonzalez placed them on the conveyor belt. The hosts were far too attack-minded right from the off and their criminally high defensive line against a trident of Ronaldo, Benzema and Gareth Bale bordered the inexcusable.
In his defence, a number of his players did little to stem the tide despite the questionable tactics, and Alvaro Gonzalez both conceded a needless penalty and lost the run of Ronaldo for his third minutes later. Real scored another painfully simple goal through Benzema on the half-hour mark but, even at 4-0 down, Espanyol had chances and Gerard Moreno spurned two excellent ones before the break.
The second half threw up signs of slight improvement, with Real goalkeeper Keylor Navas forced into a couple of smart saves, but at the other end there was still a clear lack of organisation and they shipped two more soft goals. There were positives and negatives for the hosts, who, on another day, could have scored three or four themselves, but the negatives heavily outweigh anything positive unfortunately.
Real Madrid's performance
Twenty minutes was all Ronaldo needed to remind his doubters that class is permanent. Without a goal in his last seven for club and country, Ronaldo - who scored 61 times last season - came into the encounter with a question hanging over his capacity to continue scaling those goalscoring heights. He answered it.
With seven minutes on the clock, the Portuguese capitalised on Espanyol's high defensive line before slotting past Pau Lopez. Two more followed in the form of a penalty and a tap-in to convert to Bale's wonderful cross, before then setting up Benzema for the fourth.
After the break, Madrid, who return to action in the Champions League next week, took their foot off the gas, with their defence and Navas suddenly being asked all of the questions but they survived. Incidentally, Navas became the first Real Madrid goalkeeper in 40 years to keep clean sheets in the first three Liga outings of a season - and he certainly worked hard to earn the distinction.
There always seemed to be more goals for the visitors if they wanted them, however, and Ronaldo picked up his fourth when Bale set him up on 62 minutes. Yet again, the defending was suspect. And again 10 minutes before the end when Lucas Vazquez skipped by Joan Jordan with ease to set up the talisman for another tap-in. Four of his five goals were scored with his first touch.
Despite the perceived ease in which he scored, though, it was the Cristiano Ronaldo show at Real once again and a hard-hitting reminder of his quality. As if it was ever really in doubt.
Sports Mole's man of the match
Cristiano Ronaldo: With five goals and an assist, there can simply be no other option. It was not classic Ronaldo by way of all-round performance, but few can argue that he is not as prolific as ever.
It is a case of take your pick of incidents - but both candidates must be Alvaro. First, he fouled Bale - whose first touch was heading out for a goal kick - to concede the penalty, before then allowing Ronaldo to nip in ahead of him for the third. Honourable mention to Moreno, who missed a host of chances.
Referee Inaki Bikandi Garrido was called into action early on to award the visitors the penalty, which proved a right call, and had no other major decisions to make from that point on. He also booked four players and all looked correct.
Espanyol: Sergio's side make the trip to the Anoeta Stadium to face Real Sociedad next Saturday night.
Real Madrid: Benitez and co return to Madrid now and prepare to host Shakhtar Donetsk in their Group A opener of the Champions League on Tuesday.