Objective: College students with attention deficit/hyperactivity disorder (ADHD) are at increased risk for numerous educational and psychosocial difficulties. This study reports findings from a large, multisite randomized controlled trial examining the efficacy of a treatment for this population, known as ACCESS -- Accessing Campus Connections and Empowering Student Success. Method: ACCESS is a cognitive--behavioral therapy program delivered via group treatment and individual mentoring across two semesters. A total of 250 students (18-30 years of age, 66% female, 6.8% Latino, 66.3% Caucasian) with rigorously defined ADHD and comorbidity status were recruited from two public universities and randomly assigned to receive ACCESS immediately or on a 1-year delayed basis. Treatment response was assessed on three occasions, addressing primary (i.e., ADHD, executive functioning, depression, anxiety) and secondary (i.e., clinical change mechanisms, service utilization) outcomes. Results: Latent growth curve modeling (LGCM) revealed significantly greater improvements among immediate ACCESS participants in terms of ADHD symptoms, executive functioning, clinical change mechanisms, and use of disability accommodations, representing medium to large effects (Cohen's d, 0.39 - 1.21). Across these same outcomes, clinical significance analyses using reliable change indices (RCI; Jacobson & Truax, 1992) revealed significantly higher percentages of ACCESS participants showing improvement. Although treatment-induced improvements in depression and anxiety were not evident from LGCM, RCI analyses indicated that immediate ACCESS participants were less likely to report a worsening in depression/anxiety symptoms. Conclusions: Findings from this RCT provide strong evidence in support of the efficacy and feasibility of ACCESS as a treatment for young adults with ADHD attending college.