Prader-Willi syndrome (PWS) is a genetic neurodevelopmental disorder of life-threatening hyperphagia, obesity, intellectual deficits, compulsivity, and other behavioral problems. The efficacy and safety of i.n. carbetocin, an oxytocin analog, was evaluated in a prospective, randomized, double-blinded trial in adolescents with PWS. Eligible patients aged 10-18 years with genetically confirmed PWS were randomized (1:1) to i.n. carbetocin or placebo 3 times daily for 14 days. The primary efficacy endpoint was change in parent/caregiver-rated Hyperphagia in PWS Questionnaire-Responsiveness (HPWSQ-R) total score. Secondary efficacy endpoints included HPWSQ-R behavior, drive, and severity domains; clinician-rated HPWSQ; Children's Yale-Brown Obsessive-Compulsive Severity Scale; food domain of the Reiss Profile; and Clinical Global Impression-Improvement scale. Endpoints were assessed using analysis of covariance. Relationship between primary and secondary endpoints was assessed using Pearson correlation coefficients. Safety was assessed throughout the study. Demographics and clinical characteristics were similar between treatment groups (carbetocin, n = 17; placebo, n = 20). Patients receiving carbetocin had statistically significant reductions in HPWSQ-R total score at study end (-15.6) versus patients receiving placebo (-8.9; P = 0.029); several secondary efficacy endpoints also demonstrated significant differences (P < 0.05). Treatment effects for the primary and secondary endpoints were highly correlated (P ≤ 0.0001). Incidence of adverse events (AEs) was similar between treatment groups. I.n. carbetocin was well tolerated and improved hyperphagia and behavioral symptoms of PWS. ClinicalTrials.gov: NCT01968187FUNDING. The study was funded by Ferring Pharmaceuticals. Recruitment was aided by ongoing work in PWS performed through Eunice Kennedy Shriver National Institute of Child Health and Human Development grant U54 HD083211.