Risankizumab is a fully humanized monoclonal antibody of the IgG1 subclass directed towards IL (interleukin)-23p19. Risankizumab binds with high affinity to human IL-23 and inhibits IL-23 stimulated IL-17 production. This is a Phase 2b/3, multicenter, randomized, double-blind, placebo-controlled study designed to evaluate the efficacy and safety of risankizumab as induction therapy in adult subjects with moderately to severely active UC, defined as Adapted Mayo score of 5 – 9 points (using the Mayo scoring system, excluding Physician’s Global Assessment) with an endoscopic subscore of 2 or 3 on screening endoscopy, confirmed by central review. The study will enroll patients who have had intolerance or inadequate response (IR) to prior biologic therapy (bio-IR). The bio-IR population is defined as subjects with documented intolerance or inadequate response to one or more of the approved biologics for UC (infliximab, adalimumab, golimumab, and/or vedolizumab). This Phase 2b/3 study has an operationally seamless design and comprises two sub-studies. The purpose of this design is to seamlessly transition from the Phase 2b induction study to the Phase 3 induction study without enrollment pause. Sub-Study 1 is designed as a Phase 2b dose finding study and will evaluate the efficacy, safety, and PK of risankizumab as induction treatment to identify the appropriate induction dose of risankizumab for further evaluation in Sub-study 2. After approximately 240 subjects are randomized and completed Week 12 treatment, the dose selection analysis will be conducted. During analysis of Sub-Study 1, subjects can continue to enroll in the highest dosing arm (risankizumab 1800 mg IV Weeks 0, 4, 8) on an open-label basis. Once an induction dose is selected, Sub-Study 2 will begin enrollment. Sub-Study 2 is a Phase 3 induction study to evaluate the efficacy and safety of risankizumab versus placebo. The dose selected for Sub-Study 2 will not exceed the doses evaluated in Sub-Study 1. Approximately 480 subjects will be randomized in the double-blind portion of Sub-Study 2.