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Background: As treatment of choice in promoting psychological flexibility, Acceptance and Commitment Therapy (ACT) was found to be effective in several conditions, and among different populations, including weight management in individuals with obesity. However, the mechanism of action of psychological flexibility is less known. The aim of the present study is, within the context of a brief ACT intervention for behavioral change and behavioral maintenance of a healthy lifestyle in a sample of inpatients with obesity, to explore the effect of each subcomponent of the psychological flexibility model on treatment processes and outcomes., Methods: A randomized controlled trial will be conducted. Ninety Italian adult inpatients with obesity attending a rehabilitation program for weight loss will be randomly allocated into three experimental conditions targeting respectively each subcomponent of the psychological flexibility model: group Engage focused on values-oriented behaviors, group Openness focused on acceptance and cognitive defusion, and group Awareness focused on being present and aware of thoughts, feelings, and behaviors at every moment. Weight, BMI (kg/m 2 ), the Psychological General Well-Being Inventory (PGWBI), the Outcome Questionnaire-45.2 (OQ-45.2), the Depression Anxiety and Stress Scale (DASS-21), the Difficulties in Emotion Regulation Scale (DERS), the Dutch Eating Behaviors Questionnaire (DEBQ), the Brief Values Inventory (BVI), the Committed Action Questionnaire (CAQ), the Italian-Cognitive Fusion Questionnaire (I-CFQ), the Five Facet Mindfulness Questionnaire (FFMQ), and the Acceptance and Action Questionnaire (AAQ-II) will be assessed at the beginning (time 0), at the end of psychological intervention (time 1), and after 3 (time 2) and 6 months (time 3) and 9 months (time 4) from discharge. During the following month after discharge, outpatients will be monitored in their adherence to a healthy lifestyle, using a wearable device. To assess the effectiveness of the intervention, mixed between-within 3 (conditions) × 4 (times) repeated measure ANOVAs will be conducted to examine changes from time 0 to time 1, 2, 3, and 4 in means of weight, BMI, and means of scores PGWBI, OQ-45.2, DASS, DERS, DEBQ, AAQ-II, BVI, CAQ, I-CFQ, and FFMQ, between three groups., Discussion: This study will contribute to clarify the mechanism of action of each subcomponent of the psychological flexibility model and understand its impact on the promotion of a healthy lifestyle., Trial Registration: ClinicalTrials.gov NCT04474509 . Registered on July 4, 2020.