After last weekend's Clasico win over second place Barcelona it appears a formality for Madrid to take the title back from their arch-rivals, and Guler helped them move one step closer.
The playmaker, 19, has endured an injury-hit season but on his first league start snatched a heavily rotated Madrid side a tight victory at the Reale Arena.
Real Madrid coach Carlo Ancelotti rested virtually his entire first choice team with Tuesday's Champions League semi-final visit to face Bayern Munich in mind.
Top scorer Jude Bellingham was on the bench after recovering from a stomach problem to earn a late addition to the squad list, alongside Vinicius Junior, Toni Kroos and various other stars. As a result Madrid conjured little in attack beyond Guler's goal, with the hosts, sixth, on top in driving San Sebastian rain.
Madrid goalkeeper Kepa Arrizabalaga, on loan from Chelsea, pushed away a Takefusa Kubo effort as La Real continued to probe. Ancelotti also gave defender Eder Militao his first start after his long-term knee injury.
The Brazilian looked uncertain but Real Sociedad could not capitalise, with Benat Turrientes firing narrowly over. Despite previously failing to generate openings of their own, Madrid took the lead in the 29th minute.
Aurelien Tchouameni swept the ball out to Dani Carvajal on the right, who stroked a low ball across the area. It was a perfectly placed pass by the Spain international - playing seemingly only as he is suspended for the visit to Munich - and Guler arrived in the right place at the right time to finish.
Kubo thought he had pulled Real Sociedad level against his former side before the break when he beat Militao and slotted home, but Ander Barrenetxea was penalised for a foul on Tchouameni in the
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