Interest in the association between patient and therapist's motion synchrony and the working alliance has been growing in recent years. This interest is part of a larger effort in psychotherapy research to study how the working alliance, being central to the therapeutic process, develops over the course of therapy. However, while previous studies suggest that such an association between motion synchrony and the working alliance exists, there are mixed results regarding the direction of it. The present single-case study seeks to shed light on these mixed results with a multimodal perspective of nonverbal synchrony. That is, through an exploration of a single case, the present study explores physiological synchrony as an indicator of context in which motion synchrony is associated with the working alliance. For this aim, a single case was chosen from a randomized control trial investigating short-term psychodynamic treatment for major depressive disorder. Statistical analysis identified an interaction between physiological synchrony and motion synchrony in predicting working alliance levels. Findings show that in the context of an antiphase pattern of physiological synchrony (negative association between physiological measures of the two participants), there was a positive association between motion synchrony and the working alliance. This study emphasizes the potential of a multimodal approach, while suggesting a possible explanation for mixed results in current literature that focuses on the association between motion synchrony and the working alliance. (PsycInfo Database Record (c) 2023 APA, all rights reserved).