Background: Treatment-resistant depression (TRD) represents a significant clinical challenge, affecting approximately 30% of individuals with major depressive disorder who fail to respond to conventional antidepressant pharmacotherapy. Cognitive Behavioral Therapy (CBT) has emerged as a promising intervention for this population, yet the specific mechanisms and long-term efficacy remain incompletely understood. Objective: This study systematically examines the efficacy of CBT in treating TRD, investigating psychological and neurobiological mechanisms of change and evaluating long-term outcomes.

Methods: A comprehensive review was conducted of randomized controlled trials, meta-analyses, and key observational studies published between 2000 and 2025, identified from PubMed, PsycINFO, and Cochrane Library databases. Special attention was given to studies examining CBT as an augmentation strategy, sudden gains in treatment response, and neural correlates of therapeutic change.

Results: Robust evidence supports CBT as an effective augmentation strategy for TRD, with effect sizes comparable to pharmacological augmentation approaches (ES = 1.58, 95% CI: 1.09–2.07). Psychological flexibility emerged as a key mechanism predicting long-term outcomes at 12-month follow-up. Neuroimaging findings reveal both treatment-responsive prefrontal and parietal regions and treatment-resistant cerebellar regions, suggesting that CBT may not uniformly normalize all neural abnormalities. Long-term relapse rates among TRD patients receiving CBT augmentation range from 25-50% at 12 months, with the greatest durability observed in those who achieve sudden therapeutic gains early in treatment.

Conclusion: CBT constitutes an essential component of the TRD treatment arsenal, offering durable benefits beyond pharmacotherapy alone. Future research must prioritize standardized TRD definitions, biomarker-driven personalization, and scalable delivery models to optimize clinical implementation.