BACKGROUND AND OBJECTIVE: Depressive disorders are common mental illnesses associated with high burden of disease. In this study, the effects of an online group behavioral activation on depressive symptoms and rumination were evaluated, and trajectories of change in patients with major depressive disorder adapted for a lower-middle income context investigated. METHODS: This study was an online single-group non-randomized trial. Patients were recruited from various parts of the country through social media and medical university clinics. In total, 79 women with major depressive disorder were included. We measured Beck Depressive Inventory-II (BDI-II) scores, depressive rumination and behavioral activation scores, sudden gains, depression spikes, early response, and clinical change. Patients attended an online weekly ten-session behavioral activation therapy. K-nearest neighbor was used to impute missing data and estimate the importance of candidate predictors of clinical change. However, due to a high attrition rate, paired tests were performed using per-protocol analysis without data imputation. RESULTS: The mean difference (SD) of the BDI-II score from the beginning to the end of the study was 21.10 (10.21), P <
0.001. Improvement was observed for depressive rumination (P <
0.001), and for behavioral activation (P <
0.001). Clinical change and early response were significantly related to BDI-II change (both P <
0.001). Favorable changes in behavioral activation, depressive symptoms, or rumination showed linear patterns. Clinical change (P = 0.453) and BDI-II (P = 0.050) were not statistically different between patients with moderate versus severe symptoms. LIMITATIONS: Participants were solely women. Some patients did not attend all sessions, and participants were not followed in the post-treatment period. CONCLUSION: online group behavioral activation therapy is suggested as an appropriate and accessible front-line treatment for moderate to severe major depressive disorder in lower-middle income countries.