This report presents the case of a 60-year-old woman who was diagnosed with stage IV lung adenocarcinoma with asymptomatic brain metastases and commenced chemotherapy with cisplatin/pemetrexed (CDDP/Pem). She experienced tonic-clonic convulsions on day 9 of the first cycle, which were accompanied by increased blood pressure (173/69 mm Hg) and headache. Therefore, brain MRI was performed to check for stroke or progression of brain metastatic foci. T2-weighted, FLAIR, and ADC map images showed high-intensity areas in the subcortical region of the bilateral parieto-occipital lobes, leading to a diagnosis of posterior reversible encephalopathy syndrome (PRES). The symptoms improved after treatment with antihypertensive and antiepileptic drugs. Clinicians should keep it in mind that central nervous system symptoms during anticancer therapy containing Pem may indicate possible PRES.