Objective To investigate the relationship between body mass index (BMI) and obesity-related cancers in men and women with type 2 diabetes (T2D). Design Observational cohort study. Setting Primary care. Participants A total of 52 044 patients with T2D who participated in the ZODIAC (Zwolle Outpatient Diabetes project Integrating Available Care) study between 1998 and 2012 was included (49% women). A dataset of these patients was linked to available information of the Netherlands Cancer Registry to obtain data on cancer incidents. Primary outcome measures Analyses were performed for the total group of obesity-related cancers and for non-sex-specific and sex-specific obesity-related cancers (in men: advanced prostate cancer, in women: ovarian, endometrial and postmenopausal breast cancer). Results The median follow-up period in all analyses was 3.1 (1.7–5.0) years in men and 3.1 (1.7–5.1) in women. During follow-up, 689 men and 914 women were diagnosed with an obesity-related cancer. In men, BMI was associated with a higher risk of the total group of obesity-related cancers and non-sex-specific obesity-related cancers (HR (per 5 kg/m 2 increase) 1.12 (95% CI 1.02 to 1.23) and HR 1.18 (95% CI 1.06 to 1.31)). No association was found with prostate cancer. In women, an association between BMI and all obesity-related cancers combined and sex-specific obesity-related cancers was present (HR 1.15 (95% CI 1.08 to 1.22) and HR 1.22 (95% CI 1.14 to 1.32)). No association with non-sex-specific cancers was found in women. Conclusions BMI is associated with obesity-related cancers in men with T2D, except with advanced prostate cancer. The results of this study provide reason to reconsider the classification of advanced prostate cancer as an obesity-related cancer, at least in T2D. In women, BMI is associated with the total group of obesity-related cancers and with sex-specific obesity-related cancers.