We report a case of a well-differentiated fetal adenocarcinoma (WDFA) of the lung in a 50-year-old male smoker. The literature regarding this uncommon tumor is limited. This rare variant of adenocarcinoma is more common in females, in the third and fourth decades. Microscopically it is composed of neoplastic glands and tubules that resemble the fetal lung. Well-differentiated fetal adenocarcinoma is a low-grade malignancy and surgery is the preferred mode of therapy. This uncommon case of a well-differentiated fetal adenocarcinoma in an adult male patient is reported, with relevant immunohistochemical findings, along with a discussion of the current literature.