<p>Therapeutic alliance is a well-known predictor of psychotherapy outcome and is often measured by subjective self-reports prone to biases. This pilot study examined whether interpersonal synchrony (IPS) in movement and heart rate (HR), derived from wearable wristbands, can serve as an objective measure of therapeutic engagement in naturalistic cognitive behavioral therapy sessions. Twelve transdiagnostic patient-therapist dyads were analyzed in a short-term follow-up design. Physiological signals were continuously recorded with multisensor wrist-worn devices, while head and body movements were extracted from video recordings. After each session, both dyad members completed alliance questionnaires, and patients provided standardized symptom ratings. We quantified IPS for both modalities by lagged-windowed cross-correlations. In-phase HR synchrony during the initial session significantly predicted changes in patients’ global severity index and depressive symptomatology over time. Notably, only therapists’ alliance ratings were associated with short-term outcomes. While HR data obtained from wristband sensors show potential for assessing interpersonal physiological coupling, results remain preliminary given the small, artifact-filtered subsample analyzed. The convenience and unobtrusive nature of wearable technology allows low-threshold collection of objective data even in the sensitive setting of psychotherapy. However, this pilot study indicates that convenience has to be weighed against data quality. Achieving higher precision with high-resolution devices may require greater intrusiveness. Establishing more consistent methodological standards are essential to advance IPS-based approaches towards characterizing and individualizing evidence-based psychotherapy.</p>