<i>Background:</i> In order to improve the prognosis of gastric cancer patients, the timely identification of second primary cancers is considered to be a crucial clinical problem. <i>Methods:</i> We analyzed the clinicopathological data of 2,250 patients with gastric cancer with regard to both synchronous and metachronous second primary cancers. <i>Results:</i> Of 2,250 patients, 95 (4.2%) had a second primary cancer. Both colorectal and lung cancer were frequently detected, followed by cancer in the liver, esophagus and breast. Regarding the time of detection for such second cancers, 65% of colorectal cancers were detected synchronously, while more than 80% of lung cancers were detected metachronously. The prognosis of gastric cancer patients with a second primary cancer was more negatively influenced by a second primary cancer than by a primary gastric cancer. <i>Conclusion:</i> Since gastric cancer patients may develop synchronous and metachronous second cancers in other organs, effective preoperative and postoperative diagnostic modalities both for second primary cancers, as well as for the recurrence of gastric cancer, need to be developed.