The Portugal captain had failed to find the net in Real's opening two games of the campaign but a blistering performance took his tally in Spain's top flight to 230, overtaking former striker Raul's total of 228.
The 30-year-old FIFA Ballon d'Or holder scored in the seventh, 17th (penalty), 20th, 61st and 81st minutes and Karim Benzema struck in the 28th minute as Real put a hapless Espanyol to the sword to move on to seven points from three matches.
Ronaldo, who hit five against Granada in La Liga last season, raced clear and finished calmly to set Real on their way before converting a penalty won by Gareth Bale.
Bale's centre from the left set up Ronaldo to volley in his third, equalling Barcelona forward Lionel Messi's Spanish record of 32 hat-tricks, before he turned provider by crossing for Benzema to make it 4-0 inside half an hour.
Bale set Ronaldo up again after a swift counter attack and substitute Lucas Vazquez made the assist for the former Manchester United man's fifth and Real's sixth.
"They (Espanyol) have a good team but when Cristiano has his day it's impossible," Real's Portugal defender Pepe told Spanish television.
Champions Barcelona play at 2013-14 title winners Atletico Madrid later on Saturday with both teams having won their opening two games without conceding a goal.

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