ObjectiveIn 2021, centres across all seven NHS-England regions were selected to develop pilot clinics with the aim of treating children and young people (CYP) living with complications relating to excess weight (CEW). We led a process to develop core outcomes to enable the evaluation of these clinics.MethodsA two-round Delphi process, virtual steering group meetings and two patient representation workshops were used to agree the most important outcomes for both clinicians/allied professionals and representative prospective service users.ResultsA total of 119 clinicians/allied professionals were invited to contribute to the Delphi process: 62 (52%) agreed and completed round 1 and 47 of these (76%) went on to complete round 2. Six young people (age range 13–17 years) and six parents were involved in two patient representation workshops and their experiences fed into virtual steering group meetings, via a representative.There were 44 outcomes assessed in round 1 and 21 outcomes assessed in round 2. There were 16 core outcomes selected: anthropometric, glucose tolerance/insulin resistance/type 2 diabetes, blood pressure, lipid profile, breathing problems, identification of aetiology, non-alcoholic fatty liver disease, idiopathic intracranial hypertension, anxiety, depression, self-esteem, quality of life, school attendance, dietary habits including disordered eating, exercise and activity habits.ConclusionsUse of an online Delphi process, patient representation workshops and virtual steering group meetings has enabled the development of core outcomes for clinical obesity services with eight physical health, five mental health and three self-management outcomes. Further work is needed to develop outcome measures to complete a core outcome set. These will be used to guide the evaluation of novel regional clinics for the treatment of complications of excess weight.