Immune checkpoint inhibitors (ICIs) have shown promise in treating cancer patients, and pembrolizumab is a monoclonal IgG4 antibody that targets a human cell surface protein (receptor) called PD-1. Among the side effects, a rare cranial nerve palsy unrelated to the surgical treatment may occur. We report a case of a woman, which after neurosurgical treatment for cerebellar metastasis presented painless third cranial nerve palsy. The benefits of ICIs have been ascertained, but side effects also take place. Neurological symptoms should be recognized early to avoid substantial morbidity, and if necessary, the oncologic treatment should be changed.