Background The use of parent-focused internet-based programs for the prevention and treatment of childhood obesity has shown promise but there is an acknowledged gap in parent-focused interventions which target the early childhood stage. Methods The aim of this study is to determine the efficacy of Time2bHealthy - an online healthy lifestyle program for parents of preschool-aged children. The program will be evaluated using a two-arm, parallel, randomised controlled design. The 11-week program is underpinned by Social Cognitive Theory and consists of interactive modules on healthy eating, physical activity, screen-time and sleep. The intervention also involves elements of social media, where participants share discuss ideas and experiences and they can interact and obtain information with experts. Time2bHealthy is being compared to a comparison condition. Outcomes include change in BMI (primary outcome), dietary intake, physical activity, sleep, child feeding, parental role-modelling and parent self-efficacy. Process evaluation data, such as adherence and engagement with the online forums, will also be collected. Discussion Time2bHealthy is the first randomised controlled trial to our knowledge to assess the efficacy of an online parent-focussed healthy lifestyle program for preschool-aged children in changing body mass index. Early childhood is a crucial time for establishing healthy lifestyle behaviours and parents play an important role. This study therefore fills an important gap in the literature. If found to be efficacious, Time2bHealthy has potential for broad-reach access and translation into primary health care services. Trial Registration The study is registered with the Australian and New Zealand Clinical Trials Registry (12616000119493).