BACKGROUND: Obesity is a major health problem in Brazil affecting 19% of Brazilian adults with a rising incidence over the last 10 years. Moreover, low fruit/vegetables consumption and high sweetened beverage intake are major issues. Facing the challenge of universal healthcare access, internet-based programs have the potential to reach a large number of inhabitants, be widely accessible and cost effective. Our aim is to to assess the efficacy of a web-based platform to promote weight loss and diet and physical activity habits change in a Brazilian adult population. METHODS: We designed a three-arm parallel randomized controlled trial including 18-60 years university students or employees with 25 kg/m2 minimum body mass index (BMI). Pregnancy, conditions with specific dietary requirements and participation in other weight loss programs are exclusion criteria. Participants are allocated to one of three groups: (1) waitlist with minimal intervention, (2) web-based platform, (3) web-based platform plus online dietician assistance. Assessors are blinded. Weight and BMI loss are the primary outcomes. Diet and physical activity behaviours, health perception and online activity features are secondary outcomes. The intervention comprises recommendations of diet and physical activity habits tailored to the Brazilian population and principles of behaviour change. The web-based platform has online social network and gamification features. Analysis will be on an intention-to-treat basis at 12 and 24 weeks after baseline. Differences in weight loss between groups will be performed by analysis of covariate. Linear regression will be used to assess whether treatment group allocation is an independent predictor of weight loss. The study was approved by the Federal University of Minas Gerais (UFMG) Ethics Research Committee. All participants signed an informed consent form prior to recruitment. DISCUSSION: We present the study protocol of a three arm parallel randomized controlled trial which aims to test the efficacy of an online platform to promote weight loss for adults with overweight and obesity. We anticipate that the adoption of healthy lifestyle habits and weight loss will be more important in participants randomized to the online platform group. TRIAL REGISTRATION: NCT03435445 on February 16th, 2018.