The relationship between body mass index (BMI) and risk of inflammatory bowel disease (IBD) is controversial. We performed a dose-response meta-analysis to investigate the association between BMI and risk of incident ulcerative colitis (UC) and Crohn's disease (CD) using prospective cohort studies. A systematic search was conducted in MEDLINE/PubMed, SCOPUS, Cochrane, and Web of Science databases from inception to January 2019. DerSimonian and Laird random-effects model was used to estimate combined hazard ratios (HRs). Overall, 882 articles were screened, and 42 full-text articles were reviewed for inclusion using the study eligibility criteria. Five studies evaluated the association between BMI and IBD with 1 044 517 participants. Pooled results showed a significant association between participants affected by obesity and risk of CD (HR: 1.42, 95% CI: 1.18-1.71, I : 0.00). There was a significant nonlinear association between BMI and risk of CD (P = .01, coeff = 0.5024). Pooled results did not show any significant association between being underweight and risk of UC (HR: 1.07, 95% CI: 0.96-1.19, I : 0.00) or CD (HR: 1.11, 95% CI: 0.93-1.31, I : 12.8). There was no difference in the risk for UC among participants affected by obesity compared with participants categorized as having normal BMI (HR: 0.96, 95% CI: 0.80-1.14, I : 8.0). This systematic review and meta-analysis identified significant dose-response relationship between being affected by obesity, as a risk factor, and incidence of CD.