INTRODUCTION: Weight management interventions in research studies and in clinical practice differ in length, advice, frequency of meetings, staff and cost. Very few real-world programmes have published patient-related outcomes, and those that have published used different ways of reporting the information, making it impossible to compare interventions and further develop the evidence base. Developing a core outcome set for behavioural weight management programmes (BWMPs) for adults with overweight and obesity will allow different BWMPs to be compared and reveal which interventions work best for which members of the population. METHODS AND ANALYSIS: An expert group, comprised of 40 people who work in, refer to, or attend BWMPs for adults with overweight and obesity, will be asked to decide which outcomes services should report. An online Delphi process will be employed to help the group reach consensus as to which outcomes should be measured and reported, and which definitions/instruments should be used in order to do so. The first stage of the Delphi process (three rounds of questionnaires) will focus on outcomes while the second stage (three additional rounds of questionnaires) will focus on definition/instrument selection. ETHICS AND DISSEMINATION: Ethical approval for this study has been received from the University of Glasgow College of Medical, Veterinary and Life Sciences Ethics Committee. With regard to disseminating results, a report will be submitted to our funding body, the Chief Scientist Office of the Scottish Government Health Department. In addition, early findings will be shared with Public Health England and Health Scotland, and results communicated via conference presentations, peer review publication and our institutions' social media platforms.