Abstract Background: Adiposity increases endometrial cancer risk, possibly through inflammation, hyperinsulinemia, and increasing estrogens. We aimed to quantify the mediating effects of adiponectin (anti-inflammatory adipocytokine); IL6, IL1-receptor antagonist, TNF receptor 1 and 2, and C-reactive protein (inflammatory status biomarkers); C-peptide (hyperinsulinemia biomarker); and free estradiol and estrone (estrogen biomarkers) in the adiposity–endometrial cancer link in postmenopausal women. Methods: We used data from a case–control study within the European Prospective Investigation into Cancer and Nutrition (EPIC). Eligible women did not have cancer, hysterectomy, and diabetes; did not use oral contraceptives or hormone therapy; and were postmenopausal at recruitment. Mediating pathways from adiposity to endometrial cancer were investigated by estimating natural indirect (NIE) and direct (NDE) effects using sequential mediation analysis. Results: The study included 163 cases and 306 controls. The adjusted OR for endometrial cancer for body mass index (BMI) ≥30 versus ≥18.5−<25 kg/m2 was 2.51 (95% confidence interval, 1.26–5.02). The ORsNIE were 1.95 (1.01–3.74) through all biomarkers [72% proportion mediated (PM)] decomposed as: 1.35 (1.06–1.73) through pathways originating with adiponectin (33% PM); 1.13 (0.71–1.80) through inflammation beyond (the potential influence of) adiponectin (13% PM); 1.05 (0.88–1.24) through C-peptide beyond adiponectin and inflammation (5% PM); and 1.22 (0.89–1.67) through estrogens beyond preceding biomarkers (21% PM). The ORNDE not through biomarkers was 1.29 (0.54–3.09). Waist circumference gave similar results. Conclusions: Reduced adiponectin and increased inflammatory biomarkers, C-peptide, and estrogens mediated approximately 70% of increased odds of endometrial cancer in women with obesity versus normal weight. Impact: If replicated, these results could have implications for identifying targets for intervention to reduce endometrial cancer risk in women with obesity.