Obesity and being overweight are very important public health issues due to their increasing prevalence worldwide. Third-wave cognitive behavioural therapies (3wCBT) have emerged in the last few years to promote weight loss. However, the scientific evidence identifying the most commonly used 3wCBT in weight-loss interventions in adults is still needed. The objective of this scoping review will be to identify the most widely researched 3wCBT used to facilitate weight loss in an adult population who are overweight and obese, according to the published scientific literature. The search will be carried out independently by two authors in PubMed (MEDLINE), Scopus, EMBASE, Web of Science, and PsycINFO, using search equations that contain keywords related to our search question: (1) population: adult and elderly population, (2) intervention: terms related to 3wCBT, and (3) results: weight loss or weight management. The data extraction will be performed following the indications of the Cochrane manual, and the results will be presented in three tables. The 3wCBTs have shown promising results for weight loss, but it is not yet known which of them is the most widely used to achieve weight loss in the adult population. Thus, the results of this scoping review could guide professionals in the psychological treatment of obesity and being overweight.