We report the case of a 61-year-old female with advanced gastric cancer and mediastinal lymph node metastasis who developed chylothorax. The patient presented with cough, dyspnea and pain in the left lower limb, back, and hips. Her lower limb symptoms were attributed to cellulitis. Computed tomography revealed right-sided pleural effusion, multiple lymph node swelling, and thickening of the gastric wall. Following pleural aspiration, the effusion was identified to be chyle. Cytopathologically, numerous adenocarcinoma cells were detected in clumps, and subsequent esophagogastroduodenoscopy revealed type 3 gastric cancer. We diagnosed multiple lymph node metastases with pleural dissemination. Chemotherapy was administered; however, she eventually succumbed to disease progression. We suspected that the chylothorax resulted from the mediastinal lymph node metastasis that caused thoracic duct obstruction.