Background <p>A better understanding of how behavior change interventions work to reduce depressive symptoms, as well as their effects and potential mediators, is needed. This paper described the methods of the Vincular Project, an intervention based on Self-Determination Theory with changes in 24-h movement behaviors (physical activity, sedentary behavior, and sleep) to reduce depressive symptoms in Brazilian adults.</p> Methods <p>This is a randomized, controlled, blinded clinical trial with adults aged 20 to 59&#xa0;years, of both sexes, with depressive symptoms. The sample was intentional and non-probabilistic and participants were allocated to a control group and an intervention group. The intervention was carried out twice a week, for 16&#xa0;weeks, and consisted of face-to-face and online meetings. The activities aimed to make participants more aware of their living and health conditions, exploring 24-h movement behaviors, especially physical activity, and their relationships with depressive symptoms. The contents were different in each meeting and the activities were organized into four blocks, the first being contextualization, the second with general and specific guidelines on 24-h movement behaviors, the third with opportunities to practice physical activity in different contexts and situations, and the fourth with strategies for maintaining healthy behaviors in the short and long term. Baseline (January 2023), post-intervention (May 2023), and maintenance (November 2023) evaluations included depressive symptoms as primary outcome, and 24-h movement behaviors and basic psychological needs as secondary outcomes. The variables of autonomy, competence, and relatedness were also tested as mediators related to the effectiveness of the intervention on depressive symptoms. Depressive symptoms were measured by the Patient Health Questionnaire-9 (PHQ-9), 24-h movement behaviors by self-report and accelerometry, and basic psychological needs by the translated and adapted version of the Basic Psychological Needs in Exercise Scale (BPNES). To verify the effect of the intervention, per-protocol and intention-to-treat analyses will be conducted using generalized estimating equations (GEE). Structural equation models (SEMs) will be employed in the mediation analysis.</p> Discussion <p>The results of this trial will involve valuable information about the effect of a behavioral intervention to reduce depressive symptoms carried out in a middle-income country.</p> Trial registration <p>Brazilian Registry of Clinical Trials (ReBEC): RBR-7466htj. Approved 14 April 2023, <a href="https://ensaiosclinicos.gov.br/rg/RBR-7466htj">https://ensaiosclinicos.gov.br/rg/RBR-7466htj</a></p>