An 82-year-old woman was diagnosed with lung adenocarcinoma and underwent a right upper lobectomy in June 2017. Thereafter she engaged fully in daily living activities without any recurrence of lung cancer. However, she noted disturbance of consciousness and cognitive function decline in October 2021 and was sent to our hospital because of hydrocephalus revealed by head computed tomography scans. Cerebrospinal fluid examination revealed adenocarcinoma cells which were immunohistochemically positive for thyroid transcription factor 1. She was therefore diagnosed with leptomeningeal metastasis-related hydrocephalus due to postoperative recurrence of lung adenocarcinoma. The archive specimen obtained by surgery harbored epidermal growth factor receptor (EGFR) gene mutation, but she was unable to swallow medication. Ventriculo-peritoneal (V-P) shunt was performed, and as a result she was able to swallow medication and was treated with osimertinib. Spinal fluid examination did not detect any cancer cells after treatment with osimertinib and nine months later she had not experienced any progressive disease. In patients with EGFR mutation-positive lung cancer and leptomeningeal metastases with hydrocephalus, V-P shunt surgery followed by an EGFR tyrosine kinase inhibitor such as osimertinib may achieve long-term survival.