Epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitors (TKIs) effectively treat EGFR-mutant lung adenocarcinoma, demonstrating initial efficacy but eventually leading to acquired resistance. Small cell transformation is a rare resistance mechanism to EGFR-TKIs in lung adenocarcinoma, which can complicate clinical diagnosis and treatment. We present a patient with lung adenocarcinoma who underwent a prior pneumonectomy and adjuvant chemotherapy and was treated with osimertinib after the recurrence of lung cancer. Small cell transformation occurred approximately 20 months after starting osimertinib treatment. After this transformation, the patient underwent lung radiotherapy and cisplatin-etoposide chemotherapy, which stabilized the disease. Following the confirmation of small cell lung cancer (SCLC) via thyroid puncture, treatments with irinotecan, irinotecan plus atezolizumab, thyroid radiotherapy, adrenal radiotherapy, and head radiotherapy were sequentially administered, yet the disease continued to progress. The patient succumbed to the disease in May 2023 because of progression and organ failure, with an overall survival of 52.7 months, including 16 months post small cell transformation. This case highlights the possibility of osimertinib causing lung adenocarcinoma to transform into SCLC and underscores rebiopsies' importance in identifying resistance mechanisms to EGFR-TKIs. Increased levels of neuron-specific enolase and pro-gastrin releasing peptide can signal early transformation into SCLC.