Paraneoplastic rheumatologic syndromes are defined as clinical conditions that mimic primary rheumatic disease in the course of cancer; they generally improve with the effective treatment of underlying malignancy. Hypertrophic osteoarthropathy (HOA) is one of the paraneoplastic syndromes, and it is characterized by the combined presence of periostosis, digital clubbing, and swelling of soft tissues, skin, and joints in the distal extremities. HOA is commonly associated with intrathoracic malignancies (primary lung tumors or metastases). In this report, we discuss a patient with HOA secondary to lung adenocarcinoma, who was admitted with symmetric polyarthritis presenting as elderly onset rheumatoid arthritis. He was successfully treated with chemotherapy ablation for underlying malignancy. Anti-inflammatory drugs were also administered. HOA should be kept in mind in the differential diagnosis of inflammatory arthritis.