Signet-ring cell carcinomas are an aggressive, poorly differentiated, and highly invasive adenocarcinoma carrying a poor prognosis. Most of these tumors originate in gastrointestinal organs; however, primary lung signet-ring cell adenocarcinomas can rarely occur. Tumoral lymphatic infiltration is a complication of these tumors and can cause phenomena such as lymphangitic carcinomatosis, characterized by a nodular thickening of the pleura, pleural effusions, and mediastinal lymphadenopathies. We report a case of a 63-year-old ex-smoker with a 2-week clinical course of dyspnea and pleuritic chest pain in which a nodular thickening of the pleura and pleural effusion were documented and led to the diagnosis of a primary signet-ring cell adenocarcinoma of the lung with lymphangitic carcinomatosis. This complication has never been described in the context of a primary lung tumor of this subtype. Both entities carry a high mortality and have no therapeutical options. This report adds to the information available about them.