Large cystic brain metastases from lung cancer are rare but cause substantial central nervous system symptoms that often deprive patients of opportunities to receive anticancer therapy. There are no standard therapeutic strategies against this relentless condition. Here we report a patient with large cystic brain metastases from lung adenocarcinoma successfully controlled with Ommaya reservoir placement and subsequent gamma knife surgery (GKS). A 62-year-old Japanese man presented with left upper extremity paresis. Magnetic resonance imaging revealed large cystic masses in both cerebral hemispheres and multiple brain nodules. Computed tomography of the chest showed irregular nodular shadows in the lower lobe of the right lung with multiple swollen lymph nodes. His performance status (PS) and level of consciousness worsened rapidly. Thus, at that time, we could not perform bronchoscopy with the goal of establishing a pathological diagnosis. Intracystic placement of an Ommaya reservoir followed by GKS dramatically improved his PS and level of consciousness. We were subsequently able to perform bronchoscopy, which resulted in a diagnosis of lung adenocarcinoma with 100% positivity of programmed cell death-1 ligand-1 expression. The patient was started on a 3-week cycle of pembrolizumab. Substantial reduction in tumor size was observed after one course of pembrolizumab treatment. The patient had a partial remission. He has been still receiving pembrolizumab with long-term efficacy. In conclusion, our report suggests that aggressive Ommaya reservoir placement should be considered for large cystic metastatic brain tumors, even in patients with undiagnosed cancer, poor PS, and impaired consciousness.