To quantify the association between prediagnosis lifestyles with the risk of second primary cancers (SPCs) and survival of patients with gastric first primary cancer (FPC). We recruited 574 gastric patients from two major public hospitals in North Portugal (2001–2006). Smoking, alcohol and dietary habits in the year before FPC diagnosis were evaluated. Patients were followed up to 31 December 2011 for an SPC and to 31 May 2017 for vital status. Cox proportional hazards regression was used to estimate adjusted hazard ratios for incidence of an SPC or death. During follow-up, SPCs were diagnosed in five women and 23 men, and 409 patients died, corresponding to an estimated 10-year cumulative incidence of 5.2% for SPC and an estimated 15-year cumulative mortality of 72.1%. A significantly higher hazard ratio (95% confidence interval) for SPCs was observed in patients reporting a higher consumption of red and processed meat versus the lowest third (4.49: 1.31–15.37), and for mortality in those with heavy alcohol intake versus never drinkers (1.73: 1.00–2.99) and excess weight versus normal weight (1.31: 1.04–1.65); no other significant associations were observed according to prediagnosis lifestyle. Prediagnosis lifestyles may affect the occurrence of an SPC and survival among gastric FPC survivors in the long term.