Midfielder Isco Alarcon opened the scoring in the 26th minute, while Ronaldo headed in a free-kick just before half-time. Isco then struck again midway through the secondhalf, while Ibai Gomez scored a consolation goal for the visitors, who never really got into the game.
Although Isco and Ronaldo got on the scoresheet, it was former Tottenham midfielder Luka Modric who took the plaudits from the home fans after a man-of-the-match performance in which he pulled all the strings, while also playing a pivotal role in limiting Athletic's midfield from influencing the game.
As Madrid coach Carlo Ancelotti had announced the day before, Diego Lopez was chosen again ahead of Iker Casillas in goal, while Mesut Ozil, who acted angrily to being substituted against Granada on Monday, was left out in favour of Sami Khedira, who recovered from a knee injury.
Athletic midfielder Ander Herrera, who was the subject of a bid from Manchester United last week, was left on the substitutes bench and played no part in the game, which could fuel further transfer speculation considering he started both of the club's previous games.
It took the hosts 24 minutes before they had their first real attempt on goal, Ronaldo cutting inside from the left to force visiting goalkeeper Iago Herrerin to push the ball out. From the resulting corner Sergio Ramos headed the ball inches away from the back post and just beyond the reach of team-mate Marcelo.
A minute later, Madrid, who boast a fantastic record against Athletic in recent years, took the lead. Karim Benzema played the ball over the top and slack positioning by Andoni Iraola played Isco onside, and the midfielder was left unmarked to control and shoot past Herrerin.
That incident aside, Athletic were organised but were toothless in attack.
Their only effort in the first half was a free-kick from Benat Etxebarria which went just wide.
Madrid extended their lead in first-half stoppage-time. Angel Di Maria delivered a perfect cross for Ronaldo to head into the opposite corner and open his account for the campaign.
The Portuguese striker could have returned the favour to Di Maria early in the second half when he raced towards goal and saw the Argentinian unmarked on the right-hand side, but opted to shoot instead, seeing his effort blocked.
Isco made sure of the points with 12 minutes remaining, receiving the ball from Benzema on the area to set himself up for a volley which flew into the top corner, with the help of a deflection from an Athletic defender.
Athletic substitute Ibai pulled one back for Ernesto Valverde's side with 11 minutes remaining, but there was never any danger in them getting back into the match.
Madrid go joint top with Villarreal having taken all nine points from their opening three games, while Athletic suffered their first defeat of the season.