BACKGROUND: This study investigated the interplay between serum tumor necrosis factor-alpha (sTNF-α) and serotonin (s5-HT) levels in predicting 12-week antidepressant treatment outcomes among patients with depressive disorders. METHODS: We analyzed baseline sTNF-α and s5-HT levels in 1086 patients enrolled in a naturalistic study of stepwise antidepressant treatment. Remission was defined as achieving a Hamilton Depression Rating Scale score of 7 or less at 12 weeks. Logistic regression analyses adjusted for sociodemographic and clinical covariates were utilized to explore the relationships between biomarker levels and treatment outcomes. RESULTS: Elevated sTNF-α levels were significantly associated with non-remission at 12 weeks in patients with lower s5-HT levels. Conversely, in patients with higher s5-HT levels, sTNF-α levels did not show a significant association with remission outcomes. The interaction between sTNF-α and s5-HT levels significantly predicted remission status even after adjusting for potential confounders. CONCLUSIONS: The findings suggest that the combined assessment of immune and serotonergic biomarkers can enhance the prediction of antidepressant treatment outcomes. This underscores the potential of integrating biomarker profiles to tailor antidepressant strategies, thereby advancing personalized treatment approaches in depression. Future studies should explore the underlying mechanisms of these interactions to better understand their role in treatment responsiveness.