Ronaldo provided the assist for Lucas Vazquez's first-minute goal in a comfortable victory at the Santiago Bernabeu, going on to score his first Madrid treble of the campaign.
The Portugal superstar tucked away Marcelo's cutback with a firm right-footed finish to add Madrid's second and he was on target again before the break by heading in Luka Modric's right-wing corner.
And with Zinedine Zidane leaving his talismanic forward on for the full 90 minutes despite the looming clash with PSG, he wrapped up his hat-trick with 10 minutes to go.
Sociedad goalkeeper Geronimo Rulli spilled a Gareth Bale shot, allowing Ronaldo to collect the loose ball and slot into the open goal with his left foot.
3 - Cristiano Ronaldo is the first player to score a perfect hat-trick in La Liga this season (right foot, left foot and head). Machine. pic.twitter.com/OPU0bdQ8OR
— OptaJose (@OptaJose) February 10, 2018
Ronaldo - who has hit 15 home LaLiga goals against La Real, his favourite opponents at the Bernabeu - had a slow start to the campaign but has been resurgent since the mid-season break, scoring seven goals - more than any other player in LaLiga in 2018.
Madrid host PSG in the first leg of their last-16 tie on Wednesday, with Zidane having accepted his fate may be decided by the meeting after a poor LaLiga title defence and a shock loss to Leganes in the quarter-finals of the Copa del Rey.
Photo Getty Images. Caption: Real Madrid star Cristiano Ronaldo scores a header