Group therapy may offer a promising solution to reducing patient waiting lists for publicly funded mental health services. In this study, an individual brief cognitive behavioral therapy (BCBT) intervention was adapted for implementation in a group setting in the Spanish public mental health care system. The study was designed to test initial clinical effectiveness, acceptability, and feasibility of the group adaptation of BCBT for child anxiety. The study utilized an uncontrolled multiple-group design for 8 weeks (1 h per week). Inclusion criteria were (i) children and young adolescents between 8 and 15 years old, and (ii) a clinical diagnosis of general anxiety disorder, social phobia, and/or separation anxiety disorder. Five groups were completed (n = 33; mean age = 11 years; 42.4% females). A total of 31 (93.9%) participants completed at least 7 sessions, and follow-up data were collected for 84.9% (n = 28) of participants. Overall, anxiety symptoms were reduced after intervention on the Spence Children Anxiety Scale, Generalized Anxiety Disorder Questionnaire-IV, Social Anxiety Scale for Children-Revised, and Separation Anxiety Symptom Inventory. Our findings suggest that group BCBT was associated with beneficial treatment outcomes, was acceptable and feasible for children with anxiety in the Spanish public mental health system. Both participants and their caregivers reported satisfaction and feelings of safety with the intervention. Results underscore the need for a larger-scale hybrid effectiveness-implementation trial of BCBT in a group setting throughout more community mental health centers in different Spanish states. Such work could improve patient access to and benefit from an evidence-based treatment that works in community settings.