Second primary cancers (SPCs) are an increasing concern among cancer survivors. In Northern Portugal, gastric cancer incidence is high, and alone corresponds to one in 10 SPCs diagnosed. Therefore, this study aims to estimate the contribution of gastric SPCs to the mortality of patients with a previous cancer. A population-based cohort of nongastric first primary cancer (FPC) patients from the North Region Cancer Registry of Portugal, diagnosed between 2000 and 2006, was followed for a gastric SPC until the end of 2012, and for death by any cause until the end of 2017. Patients with a gastric SPC (85 synchronous and 215 metachronous, diagnosed within one and >1 year after the FPC, respectively) were matched (1:3, by sex, five-year age group, year and site of FPC diagnosis) to those who did not develop a gastric SPC and were alive when the corresponding SPC was diagnosed. Significantly higher hazards for mortality [hazard ratio (95% confidence interval)] were observed among patients diagnosed with a synchronous [3.74 (2.69-5.21) in males and 3.36 (2.02-5.58) in females] or metachronous [6.93 (5.41-8.87) in males and 5.93 (4.04-8.72) in females] gastric SPC. The differences in the 10-year cumulative mortality between gastric SPC patients and those with no SPC were approximately 30% for synchronous and over 40% for metachronous gastric SPCs, corresponding to a 1.5- to 2-fold higher relative risk, respectively. In conclusion, cancer patients with a gastric SPC have a nearly two-fold higher risk of death over 10 years than those with a corresponding FPC only.