Anti-γ-aminobutyric acid type β receptor (anti-GABABR) antibody-associated encephalitis is a type of autoimmune encephalitis. According to current literature, its pathogenesis is reported to be closely related to tumor factors. However, diagnosis can be difficult because of the rarity of cases, limited clinical understanding, and a lack of specificity in clinical manifestation and imaging presentation. Clinical trials have demonstrated that immunotherapy can prolong the survival of patients with small cell lung cancer; however, in some cases, immunotherapy may induce anti-GABABR antibody-associated encephalitis. Patients who develop this encephalitis during immunotherapy often delay treatment because the cause is not clearly identified. In this study, we report a case of a 61-year-old man with a confirmed diagnosis of small cell lung cancer who had acute onset of cognitive impairment and seizures after two cycles of durvalumab (AstraZeneca UK Limited) combination chemotherapy. This reaction was initially considered as an immune-related adverse event (irAE) caused by durvalumab treatment, and the patient was eventually considered to have a paraneoplastic neurological disorder caused by the primary tumor. This report raises awareness of the symptoms of cognitive impairment and seizures in patients with small cell lung cancer, and the possible adverse events associated with immunotherapy. This case also highlights the importance of detecting anti-GABABR antibodies in patients with small cell lung cancer.