Abstract Background The prevalence of overweight and obesity is on the rise worldwide with severe physical and psychosocial consequences. One of the most dangerous is hypertension. Lifestyle changes related to eating behaviour and physical activity are the critical components in the prevention and treatment of hypertension and obesity. Data indicates that the usual procedures to promote these healthy habits in health services are either insufficient or not efficient enough. Internet has been shown to be an effective tool for the implementation of lifestyle interventions based on this type of problem. This study aims to assess the efficacy of a totally self-administered online intervention programme versus the usual medical care for obese and overweight participants with hypertension (from the Spanish public health care system) to promote healthy lifestyles (eating behaviour and physical activity). Method A randomized controlled trial will be conducted with 100 patients recruited from the hypertension unit of a public hospital. Participants will be randomly assigned to one of two conditions: a) SII: a self-administered Internet-based intervention protocol; and b) MUC-medical usual care. The online intervention is an Internet-delivered, multimedia, interactive, self-administered programme, composed of nine modules designed to promote healthy eating habits and increase physical activity. The first five modules will be activated at a rate of one per week, and access for modules 5 to 9 will open every two weeks. Patients will be assessed at four points: before the intervention, after the intervention (3 months), and at 6 and 12 months (follow-up). The outcome variables will include blood pressure, and Body Mass Index, as primary outcome measures, and quality of life and other lifestyle and anthropometrical variables as secondary outcome measures. Discussion The literature highlights the need for more studies on the benefits of using the Internet to promote lifestyle interventions. This study aims to investigate the efficiency of a totally self-administered Internet − +based programme for promoting healthy habits and improving the medical indicators of a hypertensive and overweight population. Trial registration NCT02445833