Purpose Male cancer survivors have a higher risk of cancer than the general population, which might be caused by an increased prevalence of obesity or susceptibility to obesity-related carcinogenesis. We assessed the effects of obesity before the diagnosis of a first cancer on the development of secondary primary cancers (SPCs). Methods The study population consisted of 239,615 Korean male cancer survivors between January 2003 and December 2010. Incident SPCs were assessed throughout follow-up until December 2011. Cox proportional hazards models were used to calculate the hazard ratios of SPCs associated with prediagnosis body mass index (BMI), which were compared with those of first cancers in all cohort participants. Results After 1,614,583 person-years of follow-up, we observed 4,799 patients with SPC. The age-standardized incidence rate of cancer in cancer survivors was 1.1 times higher than that of the general population. We found positive linear trends between prediagnosis BMI and risk of all-combined, colorectal, liver, lymphoma, biliary tract, kidney, and obesity-related SPCs. The magnitude of the BMI-SPC risk association in male cancer survivors was stronger than that for first cancers in the general population, whereas the mean BMI was similar in both groups. In the severely obese category (BMI ≥ 30 kg/m2), the adjusted hazard ratios for SPCs among cancer survivors (1.41; 95% CI, 1.15 to 1.74) were significantly higher than those for first cancers among all cohort participants (1.12; 95% CI, 1.09 to 1.16; Pheterogeneity < .01). Conclusion Prediagnosis obesity is a risk factor for overall and individual SPCs, and the strength of the BMI-cancer association is slightly stronger in male cancer survivors than in the general population.