We report on a case of malignant pleural effusion as initial metastatic presentation of occult gastric cardia cancer in a young woman. To the best of our knowledge, this is the first report of gastric adenocarcinoma metastasized to pleura as an initial presentation. Location of cardia and signet ring cell histology may contribute to the manifestation. Utilization of PET/CT was helpful for proper diagnosis. For patients with such distinct clinical presentations, it would be appropriate to consider gastric cancer as one of the possible primary sites.