Objectives: This randomized, multicenter, placebo-controlled, phase IV study assessed the efficacy and tolerability of onabotulinumtoxinA in patients with overactive bladder. Methods: Patients were randomized 1:1 to onabotulinumtoxinA 100 U or placebo. Assessments over 12 weeks included: change from baseline in urinary incontinence (UI) episodes/day; proportions of patients who achieved 100% and 50% or greater reductions in UI episodes/day; proportion of patients using no incontinence pads in the previous 24 hours; and changes from baseline in micturition frequency, nocturia, urgency UI, Incontinence-Quality of Life, King’s Health Questionnaire, International Consultation on Incontinence Questionnaire—UI Short Form scores and time to request retreatment. Results: Significant reductions in UI episodes/day were seen with onabotulinumtoxinA versus placebo within week 1 posttreatment (−2.9 vs −2.0, P = 0.005) through week 12 (coprimary endpoint: −3.5 vs −1.6, P < 0.001). Significantly more onabotulinumtoxinA-treated patients achieved 100% (coprimary endpoint) and 50% or greater reductions in UI episodes/day. Decreases in other urinary symptoms were also seen within 1 week with onabotulinumtoxinA that continued through at least week 12. More onabotulinumtoxinA-treated versus placebo-treated patients required no incontinence pads at weeks 1 to 12, and greater improvements in quality of life measurements were seen. Time to request retreatment was significantly longer with onabotulinumtoxinA versus placebo (30.0 weeks vs 13.1 weeks; P < 0.001). No unexpected safety signals were observed. Urinary tract infection was the most commonly observed adverse event. Conclusions: Urinary symptom and quality of life improvements were observed with onabotulinumtoxinA within 1 week of treatment and were sustained for at least 12 weeks.