Most metastatic lung tumors display well-defined, round, multiple nodular shadows, whereas the presence of diffuse ground-glass opacities on chest computed tomography generally suggests non-malignant conditions. Here, we report an unusual case of pulmonary metastasis from gastric cancer in which diffuse ground-glass opacities were observed in all lung segments. A 59-year-old man with a 3-month history of worsening chest pain and shortness of breath was referred to the pulmonary clinic. Chest computed tomography revealed low attenuation areas, suggesting emphysema, along with diffuse ground-glass opacities and interlobular septal thickening in both lungs. A transbronchial lung biopsy specimen revealed signet-ring cell carcinoma infiltrating the alveolar septa. Immunohistochemical staining of the cancer cells was positive for CDX-2, cytokeratin 7, and cytokeratin 20, and negative for surfactant apoprotein-A, TTF-1, and Napsin A. Gastrointestinal endoscopy revealed an ulcerative tumor in the stomach, and a biopsy from the tumor demonstrated malignant cells with similar morphology and immunophenotypes as those in the lungs. The final diagnosis was diffuse lung metastasis from gastric cancer. Our case shows that although multiple, well-defined nodules are typically considered to be the classic presentation of pulmonary metastasis, clinicians should also be aware of the possibility of pulmonary metastasis presenting as diffuse ground-glass opacities.