Background: The role of insulin resistance as a mediator in the association of body mass index (BMI) with site-specific cancer risk has, to our knowledge, never been systematically quantified. We aimed to determine to what extent insulin resistance measured as the logarithmized triglyceride glucose product (TyG index) mediates the effect of BMI on risk of obesity-related cancers. Methods: A total of 510,471 individuals from six European cohorts with a mean age of 43.1 years were included in the study. We fitted Cox models, adjusted for relevant confounders, to investigate associations of TyG index with ten common obesity-related cancer sites, and quantified the proportion of the effect of BMI mediated through TyG index. Results: During a median follow-up of 17.2 years, 16 052 individuals developed obesity-related cancers. TyG index was associated with the risk of cancers of the kidney (hazard ratio (HR) per one standard deviation increase 1.13, 95% confidence interval: 1.07-1.20), liver (1.13, 1.04-1.23), pancreas (1.12, 1.06-1.19), colon (1.07, 1.03-1.10), and rectum (1.09, 1.04-1.14). Substantial proportions of the effect of BMI were mediated by TyG index for cancers of the pancreas (42%), rectum (34%), and colon (20%); smaller proportions for kidney (15%) and liver (11%); none for endometrium, ovary and breast (postmenopausal). Conclusions: In this pooled cohort study including more than 500,000 individuals, insulin resistance measured as the logarithmized triglyceride glucose product significantly mediated the effect of overweight and obesity on risk of cancers of the kidney, liver, pancreas, colon, and rectum. In contrast, insulin resistance did not mediate the risk for cancers of the endometrium, ovary and breast. Our results confirm a promoting role of insulin resistance in the pathogenesis of gastrointestinal cancers. Although often claimed, insulin resistance does not appear to connect excess body weight with cancers of the female reproductive organs.