Several studies have investigated associations between overweight/obesity and risk of developing rheumatoid arthritis, however, the evidence is not entirely consistent, and previous meta-analyses mainly included case-control studies, which can be affected by various biases. We therefore conducted a systematic review and meta-analysis of cohort studies on adiposity and risk of rheumatoid arthritis. Relevant studies were identified by searching PubMed and Embase databases. Random effects models were used to estimate summary relative risks (RRs) and 95% confidence intervals (CIs) for rheumatoid arthritis in relation to different measures of adiposity. Thirteen cohort studies (10 publications) were included. The summary RR per 5 kg/m2 increase in body mass index (BMI) was 1.11 (95% CI 1.05-1.18, I2 = 50%), but the association was restricted to women (1.15, 95% CI 1.08-1.21, I2 = 17%) and not observed in men (0.89, 95% CI 0.73-1.09, I2 = 58%). The summary RR per 5 kg/m2 increment in BMI at age 18 years was 1.17 (95% CI 1.01-1.36, I2 = 26%, n = 3), and per 10 cm increase in waist circumference was 1.13 (95% CI 1.02-1.25, I2 = 44%, n = 2). Higher BMI in middle age, BMI at age 18 years, and waist circumference were associated with increased rheumatoid arthritis risk, suggesting adiposity could be targeted for primary prevention.