Young adulthood is a high-risk life stage for weight gain. Evidence is needed to translate behavioural approaches into community practice to prevent weight gain in young adults. This systematic review assessed the effectiveness and reporting of external validity components in prevention interventions. The search was limited to randomized controlled trial (RCT) lifestyle interventions for the prevention of weight gain in young adults (18–35 years). Mean body weight and/or body mass index (BMI) change were the primary outcomes. External validity, quality assessment and risk of bias tools were applied to all studies. Twenty-one RCTs were identified through 14 major electronic databases. Over half of the studies were effective in the short term for significantly reducing body weight and/or BMI; however, few showed long-term maintenance. All studies lacked full reporting on external validity components. Description of the intervention components and participant attrition rates were reported by most studies. However, few studies reported the representativeness of participants, effectiveness of recruitment methods, process evaluation detail or costs. It is unclear from the information reported how to implement the interventions into community practice. Integrated reporting of intervention effectiveness and enhanced reporting of external validity components are needed for the translation and potential upscale of prevention strategies.