There are various mechanisms underlying the resistance of EGFR-mutant lung adenocarcinoma to epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs). We herein report a case of pulmonary adenocarcinoma with EGFR mutation (exon 19 deletion and T790M) that acquired resistance to osimertinib treatment because of transformation into small-cell lung carcinoma (SCLC). A 67-year-old ex-smoking woman was diagnosed with left upper lobe adenocarcinoma of clinical stage IIIA (cT2bN2M0). She was treated with chemoradiotherapy (cisplatin and vinorelbine plus radiation), gefitinib, cisplatin, and pemetrexed followed by pemetrexed maintenance therapy and erlotinib. Since a sample extracted from the metastatic lung tumor taken obtained via a transbronchial lung biopsy was found to be positive for the T790M mutation at the time of disease progression during erlotinib treatment, she received osimertinib treatment for 15 months until progressive disease. She developed resistance to osimertinib due to the histologic transformation to SCLC. Although the standard chemotherapy of carboplatin and etoposide for SCLC was administered, she died due to metastatic liver failure.