INTRODUCTION: Effective treatment for clinical obesity is available but is rarely offered by healthcare systems, which often treat complications without treating the underlying cause. The LightWAY trial will investigate the clinical benefits and harms as well as cost-effectiveness of an intensive weight loss intervention compared with existing weight management programmes for people with clinical obesity. METHODS AND ANALYSIS: LightWAY is an investigator-initiated, international, randomised, parallel-group clinical superiority trial with blinded outcome assessment. Six hundred people seeking treatment for clinical obesity (body mass index ≥35 kg/m2 with comorbidities) will be recruited in centres in the UK and Denmark and randomised 1:1 to one of two groups. The experimental group will be offered a 2-year intensive weight loss programme providing support and advice to follow a total diet replacement programme, followed by gradual transition to an energy-reduced diet in combination with increased physical activity and if needed, prescription of weight loss medication. The control group will receive usual care, typically comprising brief behavioural support for weight loss and treatment of the complications of obesity or occasionally referral to specialist weight management services. The two co-primary outcomes are cardiometabolic risk, assessed with metabolic syndrome severity Z-score, and body weight assessed at 2 years. The secondary outcomes include the Short Form-36 mental component scale, 4-metre gait speed and proportion of participants achieving ≥20% weight loss. The key adverse effects will be the proportion of participants with at least one serious adverse event, incidence of eating disorders and disproportional loss of bone mass. Incremental cost-effectiveness will be assessed over the trial period and over the lifetime through modelling. ETHICS AND DISSEMINATION: Ethical approval was granted in the UK (August 2024, 24/SC/0211) and Denmark (December 2023, H-23065222). Findings will be disseminated through peer-reviewed journals and scientific conferences and to participants in the trial and clinicians. TRIAL REGISTRATION NUMBER: NCT06321458.