The Cool Little Kids parenting group program is an effective intervention for preventing anxiety disorders in young children who are at risk because of inhibited temperament. The program has six group sessions delivered by trained psychologists to parents of 3- to 6-year-old children. An online adaptation (Cool Little Kids Online) has been developed to overcome barriers to its wide dissemination in the community. This study tested the efficacy of Cool Little Kids Online in a randomized controlled trial.A total of 433 parents of a child aged 3 to 6 years with an inhibited temperament were randomized to the online parenting program or to a 24-week waitlist. The online program has 8 interactive modules providing strategies that parents can implement with their child to manage their child's avoidant coping, reduce parental overprotection, and encourage child independence. Parents were provided telephone consultation support with a psychologist when requested. Parents completed self-report questionnaires at baseline and at 12 and 24 weeks after baseline.The intervention group showed significantly greater improvement over time in child anxiety symptoms compared to the control group (d = 0.38). The intervention group also showed greater reductions in anxiety life interference (ds = 0.33-0.35) and lower rates of anxiety disorders than the control group (40% versus 54%), but there were minimal effects on broader internalizing symptoms or overprotective parenting.Results provide empirical support for the efficacy of online delivery of the Cool Little Kids program. Online dissemination may improve access to an evidence-based prevention program for child anxiety disorders. Clinical trial registration information-Randomised Controlled Trial of Cool Little Kids Online: A Parenting Program to Prevent Anxiety Problems in Young Children; http://www.anzctr.org.au/; 12615000217505.