Background Anxiety is the most common childhood mental health condition and is associated with impaired child outcomes, including increased risk of mental health difficulties in adulthood. Anxiety runs in families: when a parent has anxiety, their child has a 50% higher chance of developing it themselves. Environmental factors are predominant in the intergenerational transmission of anxiety and, of these, parenting processes play a major role. Interventions that target parents to support them to limit the impact of any anxiogenic parenting behaviors are associated with reduced anxiety in their children. A brief UK-based group intervention delivered to parents within the UK National Health Service led to a 16% reduction in children meeting the criteria for an anxiety disorder. However, this intervention is not widely accessible. To widen access, a 9-module web-based version of this intervention has been developed. This course comprises psychoeducation and home practice delivered through text, video, animations, and practice tasks. Objective This study seeks to evaluate the feasibility of delivering this web-based intervention and assess its effectiveness in reducing child anxiety symptoms. Methods This is the protocol for a randomized controlled trial (RCT) of a community sample of 1754 parents with self-identified high levels of anxiety with a child aged 2-11 years. Parents in the intervention arm will receive access to the web-based course, which they undertake at a self-determined rate. The control arm receives no intervention. Follow-up data collection is at months 6 and months 9-21. Intention-to-treat analysis will be conducted on outcomes including child anxiety, child mental health symptoms, and well-being; parental anxiety and well-being; and parenting behaviors. Results Funding was received in April 2020, and recruitment started in February 2021 and is projected to end in October 2022. A total of 1350 participants have been recruited as of May 2022. Conclusions The results of this RCT will provide evidence on the utility of a web-based course in preventing intergenerational transmission of anxiety and increase the understanding of familial anxiety. Trial Registration ClinicalTrials.gov NCT04755933; https://clinicaltrials.gov/ct2/show/NCT04755933 International Registered Report Identifier (IRRID) DERR1-10.2196/40707