Signet ring cell carcinoma (SRCC) of the lung is a rare variant of pulmonary adenocarcinoma. Because the majority of SRCCs seen in the lung are metastatic tumors from stomach, colon, or breast, the differentiation of primary SRCC from metastatic SRCC is important but may be problematic. Recently, immunohistochemical studies are known to be valuable in determining primary sites of SRCC. Herein, we presented a case of primary signet ring cell carcinoma of the lung in a 67-year-old man. Even though radiographic findings of our case were more suggestive of metastatic orgin of SRCC in the lung, we could finally conclude that lung was the primary site of SRCC in this case with the help of immunohistochemical studies (TTF-1 and CK7 positive and CK 20 negative) and other diagnostic work up.