New Granada boss Lucas Alcaraz enjoyed a dream first game in charge as his side pulled off a shock victory over Madrid at the Estadio Nuevo Los Carmenes.
Alcaraz took over this week following the sacking of Juan Antonio Anquela, who left with the club in 17th place in the standings, two points above the relegation zone.
Osasuna's win over Celta Vigo earlier on Saturday meant Granada had dropped into the bottom three by kick-off, but they climbed out again thanks to the own goal from Ronaldo.
Karim Benzema's horrible miss six minutes from time summed up a poor evening for Madrid, who struggled to create many clear-cut chances throughout the game.
The only goal of the game came after 22 minutes when Alvaro Arbeloa turned a dangerous cross from Carlos Aranda behind for a corner.
Gabriel Torje swung in a dangerous delivery towards the near post and the ball skimmed off Ronaldo's head as he tried to clear and into the far corner.
Things were not going much better for Ronaldo at the other end of the pitch either, with nothing coming from two of his trademark free-kicks.
The first was fired wide and the second straight into the wall as Real rarely threatened to find an equaliser.
It was no surprise that manager Jose Mourinho made two changes at the interval, with Benzema coming on for Sami Khedira and Jose Maria Callejon replacing Gonzalo Higuain.
Benzema almost got on the end of a good Ronaldo cross after 55 minutes, while the former Manchester United man then sent a weak shot trundling straight at goalkeeper Antonio Tono.
But proof that it was not going to be the away side's night arrived just before the end when Benzema embarrassingly side-footed wide when it looked easier to score after Callejon's shot had been well saved by Tono.