Desmoid tumors are rare, locally aggressive benign tumors with a high rate of recurrence even after complete resection. Only twenty percent are intrathoracic and they are often painless or minimally painful with slow growth. Rarely they can grow quite rapidly, mimicking a malignant tumor. Here we present a rare case of a rapidly growing, intrathoracic desmoid tumor mimicking lung malignancy in an elderly male ex-smoker. This case demonstrates the importance of including desmoid tumors in the differential diagnosis of lung masses, raising awareness of benign tumors mimicking malignant behavior, and emphasizing the potential benefit of timely follow-up and early diagnosis.