Lung adenocarcinoma develops after the neoplastic transformation of pneumocytes or bronchial epithelium. It is chronic and slowly progressive in nature and the clinical signs are only seen in cases that have developed tumours of considerable size. There has been no report of lung adenocarcinoma in game animals. This case study reports a lung adenocarcinoma in a 4-year-old dead roe deer (Capreolus capreolus) from the Forest District Piaski, Greater Poland Province of Poland. During necropsy, a large mass measuring 15 × 20 cm was found in the caudal lobe of the right lung. The tumour was of grey or pinkish-grey colour, solid texture and had a clear delineation between the affected and non-affected areas. In the remaining lobes, numerous small nodules of 2-20 mm in diameter with a similar structure were found. A great deal of mucoid and mucopurulent fluid throughout the bronchial tree was observed. The thoracic lymph nodes were found enlarged. The histological analysis identified the large mass found as bronchoalveolar papillary lung adenocarcinoma. The PCR for JSRV was performed and the result was negative. To the best of our knowledge, this is the first report of the lung adenocarcinoma in a roe deer.