<p>Numerous adaptations to interventions have been included in cognitive behavioral therapy (CBT) for autistic youth. This study examines the degree to which CBT adapted to the social needs of autistic youth confers significant benefit by promoting social responsiveness. A secondary analysis was conducted on a multisite randomized clinical trial (Wood et al. in JAMA Psychiatry 77:474–483, 2020) comparing adapted CBT with standard-of-practice CBT and treatment-as-usual (TA). Autistic youth (<i>N</i> = 167; aged 7–13) with maladaptive and interfering anxiety participated. The adapted CBT (BIACA) uses a modular format, with an emphasis on supplementing common CBT practice elements (reframing and graded exposure) with social skill supports. The primary outcome measure was the Pediatric Anxiety Rating Scale. Social responsiveness was assessed with the Social Responsiveness Scale, Second Edition. Participants’ general mental health was assessed as a secondary outcome using the Brief Problem Checklist. Mediation was tested using the SPSS PROCESS macro. Analyses suggested that the effect of adapted CBT on anxiety was mediated by its effects on social responsiveness, with a statistically significant indirect effect. Youth randomly assigned to adapted CBT exhibited better overall mental health at posttreatment compared to those randomized to the other conditions, and this effect was also mediated by improved social responsiveness. CBT adapted to address some of the social needs of autistic youth may enhance mental health outcomes by supporting social responsiveness, perhaps increasing the ease and effectiveness with which some youth can navigate potentially stressful situations such as entering and participating in group activities.</p>