The aetiological effects of attachment dimensions on therapeutic relationships, a determining factor for client experiences and outcomes, have been extensively documented in the psychotherapeutic literature. Especially in CBT, the possible pathways through which attachment affects the working alliance is a severely under-explored area of scientific inquiry. Hence, the present study sought to examine the possible mediating role of therapeutic reactance in the attachment dimensions-working alliance link. In a sample of 224 CBT clients (182 women and 42 men) with a mean age of 25.2 years (SD=3,87), the Experiences in Close Relationships-Revised (ECR-R), Working Alliance Inventory-Client (WAI-C) and Therapeutic Reactance Scale (TRS) were distributed, and Structural Equation Modelling (SEM) procedures revealed that therapeutic reactance fully mediates the attachment-working alliance link with an indirect effect of β=-.080, t=-3.402, p<.05 and it partially mediates the attachment avoidance-working alliance link with β=-.054, t=-2.539, p<.05 explaining 19.5% and 15% of the variance in the effect of each attachment dimension on working alliance, respectively. Findings suggest that therapeutic reactance effectively captures a large proportion of the influence of adult attachment on working alliance in CBT, deeming it a promising area for further investigation with major practical applications in CBT. Future studies utilizing longitudinal, qualitative, and clinical trial methodologies could focus on reactance prevention strategies to promote better therapeutic relationships and therefore, client outcomes.