Background: Obesity may affect efficacy and safety of biologic treatments for ulcerative colitis (UC). Tofacitinib is an oral, small molecule Janus kinase inhibitor for the treatment of UC. Aims: To assess efficacy and safety of tofacitinib in patients with UC, by baseline body mass index (BMI). Methods: This post hoc analysis evaluated patients with UC receiving placebo or tofacitinib from the 8-week OCTAVE Induction 1 and 2 (NCT01465763, NCT01458951) and 52-week OCTAVE Sustain (NCT01458574) studies. Patients were stratified by BMI at OCTAVE Induction 1 and 2 baseline (<25, 25 to <30 and ≥30 kg/m2). Outcomes included remission, endoscopic improvement, clinical response, sustained steroid-free remission, Inflammatory Bowel Disease Questionnaire total score and Short Form-36 Health Survey scores. Adverse events were evaluated. Results: At Week 8 of OCTAVE Induction 1 and 2, and Week 52 of OCTAVE Sustain, higher proportions of patients receiving tofacitinib 5 or 10 mg twice daily (b.d.) achieved clinical response vs placebo, regardless of baseline BMI subgroup (all P < 0.05). Proportions of patients achieving efficacy endpoints were generally similar across BMI subgroups; in univariate and multivariate regression analyses, BMI was not a significant predictor (all P ≥ 0.05; univariate BMI [continuous] odds ratio for remission: 0.98 [95% confidence interval 0.95, 1.02]). There was no consistent trend between BMI and adverse events. Among patients receiving tofacitinib 10 mg b.d. in OCTAVE Induction 1 and 2, serious infections were numerically greater in the BMI ≥30 subgroup (3.2%) vs other subgroups (0.4%). Limitations included small patient numbers in the BMI ≥30 subgroup. Conclusions: Efficacy and safety of tofacitinib were similar in patients with UC regardless of baseline BMI.