AbstractThe examination of nonverbal synchrony has become a promising line of psychotherapy research. Although several studies have found between‐dyad associations between nonverbal synchrony and multidimensional outcomes, the findings remain heterogeneous, and within‐dyad effects remain to be investigated. The present study examines within and between effects of nonverbal synchrony on mastery, resource activation, problem actuation, and motivational clarification (Grawe's general mechanisms of change). Four‐hundred and twenty‐three videotaped sessions of 175 patients were analysed using motion energy analysis (MEA), providing values to quantify nonverbal synchrony in the patient–therapist dyad. Grawe's general mechanisms of change in psychotherapy were rated using the Inventory of Therapeutic Interventions and Skills (ITIS). On average, patient–therapist nonverbal synchrony was greater than chance. Hierarchical linear modelling revealed that nonverbal synchrony was significantly associated with higher mastery and less resource activation on the within‐dyad level. Nonverbal synchrony was not associated with problem actuation or motivational clarification, and in general, no associations were found on the between‐dyad level. The results demonstrate the importance of disentangling within and between effects of nonverbal synchrony and provide initial evidence that nonverbal synchrony is tied to the specific therapeutic strategies observed in psychotherapy sessions.