This meta-analysis examined the association between body mass index (BMI) and risk of rheumatoid arthritis (RA) based on 20 observational studies with a total sample size of 568,889 participants. Underweight was not significantly associated with an increased risk of RA (OR: 0.84, 95% CI: 0.70–1.01, p = 0.058). Conversely, being overweight was significantly associated with an increased risk of RA (OR: 1.13, 95% CI: 1.06–1.19, p < 0.001). Obesity was also significantly associated with an increased risk of RA (OR: 1.25, 95% CI: 1.14–1.36, p < 0.001), with this association being particularly strong in women. Studies were searched in PubMed, Embase, Web of Science and the Cochrane Library from inception to September 2022. Findings were verified by sensitivity, subgroup and publication bias analyses. The study is registered under the number INPLASY2025110039.