Cognitive behavioral therapy (CBT) for child anxiety, although efficacious, typically requires 16-20 weekly sessions with a therapist. Brief CBT (BCBT; eight sessions) for child anxiety is promising but may have less favorable outcomes owing to reduced session time. Mobile health (m-health) has the potential to improve BCBT efficacy by delivering ecological momentary intervention to engage youth in learning and practicing CBT skills in their everyday lives (in vivo).We developed an m-health platform entitled SmartCAT (Smartphone-enhanced Child Anxiety Treatment). SmartCAT consists of (1) a smartphone application (app) that cues youth to use the CBT skills taught in sessions, (2) an online portal that allows therapists to monitor skill use, to send cues and treatment-related materials, to engage youth in real-time via secure messages, and to manage rewards, and (3) a communication protocol that allows real-time bidirectional exchange between the app and the portal. A pilot study with nine youth (9-14 years old) examined the platform's feasibility as an adjunct to BCBT.SmartCAT was found to be capable of supporting BCBT for child anxiety and received positive feedback from both therapists and youth patients. Patients rated the app as highly usable (mean=1.7 on a 1-7 scale, with 1="easy"). Patients completed 5.36 skills coach entries per session (standard deviation=1.95) and took an average of 3.14 min (standard deviation=0.98 min) to complete the entries.A smartphone app is feasible within CBT for child anxiety. Users found SmartCAT both acceptable and easy to use. Integrating an m-health platform within BCBT for anxious children may facilitate involvement in treatment and dissemination of effective procedures.