Importance: Urinary incontinence (UI) guidelines recommend behavioral interventions as first-line treatment using individualized approaches. A one-time, group-administered behavioral treatment (GBT) could enhance access to behavioral treatment. Objective: To compare the effectiveness, cost, and cost-effectiveness of GBT with no treatment for UI in older women. Design, Setting, and Participants: Multisite randomized clinical trial (the Group Learning Achieves Decreased Incidents of Lower Urinary Symptoms [GLADIOLUS] study), conducted from July 7, 2014, to December 31, 2016. The setting was outpatient practices at 3 academic medical centers. Community-dwelling women 55 years or older with UI were recruited by mail and screened for eligibility, including a score of 3 or higher on the International Consultation on Incontinence Questionnaire-Short Form (ICIQ-SF), symptoms of at least 3 months' duration, and absence of medical conditions or treatments that could affect continence status. Of 2171 mail respondents, 1125 were invited for clinical screening; 463 were eligible and randomized; 398 completed the 12-month study. Interventions: The GBT group received a one-time 2-hour bladder health class, supported by written materials and an audio CD. Main Outcomes and Measures: Outcomes were measured at in-person visits (at 3 and 12 months) and by mail or telephone (at 6 and 9 months). The primary outcome was the change in the ICIQ-SF score. Secondary outcome measures assessed UI severity, quality of life, perceptions of improvement, pelvic floor muscle strength, and costs. Evaluators were masked to group assignment. Results: Participants (232 in the GBT group and 231 in the control group) were aged 55 to 91 years (mean [SD] age, 64  years), and 46.2% (214 of 463) were African American. In intent-to-treat analyses, the ICIQ-SF scores for GBT were consistently lower than control across all time points but did not achieve the projected 3-point difference. At 3 months, the difference in differences was 0.96 points (95% CI, -1.51 to -0.41 points), which was statistically significant but clinically modest. The mean (SE) treatment effects at 6, 9, and 12 months were 1.36 (0.32), 2.13 (0.33), and 1.77 (0.31), respectively. Significant group differences were found at all time points in favor of GBT on all secondary outcomes except pelvic floor muscle strength. The incremental cost to achieve a treatment success was $723 at 3 months; GBT dominated at 12 months. Conclusions and Relevance: The GLADIOLUS study shows that a novel one-time GBT program is modestly effective and cost-effective for reducing UI frequency, severity, and bother and improving quality of life. Group-administered behavioral treatment is a promising first-line approach to enhancing access to noninvasive behavioral treatment for older women with UI. Trial Registration: ClinicalTrials.gov identifier: NCT02001714.