Abstract Background Insulin resistance is a hypothesised biological mechanism linking obesity with prostate cancer (PCa) death. Data in support of this hypothesis is limited. Methods We included 259,884 men from eight European cohorts, with 11,760 incident PCa’s and 1784 PCa deaths during follow-up. We used the triglyceride-glucose (TyG) index as indicator of insulin resistance. We analysed PCa cases with follow-up from PCa diagnosis, and the full cohort with follow-up from the baseline cancer-free state, thus incorporating both PCa incidence and death. We calculated hazard ratios (HR) and the proportion of the total effect of body mass index (BMI) on PCa death mediated through TyG index. Results In the PCa-case-only analysis, baseline TyG index was positively associated with PCa death (HR per 1-standard deviation: 1.11, 95% confidence interval (CI); 1.01–1.22), and mediated a substantial proportion of the baseline BMI effect on PCa death (HR_{total effect} per 5-kg/m² BMI: 1.24; 1.14–1.35, of which 28%; 4%–52%, mediated). In contrast, in the full cohort, the TyG index was not associated with PCa death (HR: 1.03; 0.94-1.13), hence did not substantially mediate the effect of BMI on PCa death. Conclusions Insulin resistance could be an important pathway through which obesity accelerates PCa progression to death.