Objectives Interpersonal dysfunction is a central feature of borderline personality disorder (BPD), and the neuropeptide oxytocin (OT) has been shown to impact patients’ behaviour in numerous ways. Nonverbal signals such as the coordination of body movement (nonverbal synchrony) are associated with the success of interpersonal exchanges and could thus be influenced by features of BPD and by the administration of OT. Design We explored the effect of intranasal OT (inOT) on nonverbal synchrony in sixteen patients with BPD and fifteen healthy controls (CTL) randomly assigned to two double‐blind clinical interviews under inOT and placebo (PL). Methods Nonverbal synchrony was assessed by automated video‐analyses of subject’s and interviewer’s body movement. Lagged cross‐correlations were used to objectively quantify coordination in dyads. Results Synchrony was higher than pseudosynchrony (= synchrony expected by chance), and there was a differential effect of inOT between groups: While healthy controls displayed increased synchrony under inOT, patients with BPD showed low levels of synchrony under inOT. Additionally, patient’s synchrony was negatively associated with self‐reported childhood trauma. Conclusions Nonverbal synchrony in clinical interviews is influenced by inOT, and this effect depends on subject’s diagnosis. In line with previous research implying positive associations between nonverbal synchrony and relationship quality, inOT led to an increase of synchrony in healthy controls, but not in patients with BPD. Low levels of synchrony under inOT in patients and its association with childhood trauma suggest that additional mechanisms such as rejection sensitivity might mediate BPD patients’ nonverbal behaviour. Practitioner points Intranasal oxytocin (inOT) attenuated nonverbal synchrony – a proxy for relationship quality – in patients with borderline personality disorder (BPD), while it increased nonverbal synchrony in healthy controls (CTL). Available models (rejection sensitivity; social salience) suggest that inOT may alter the way patients with BPD assess social situations, and this alteration is expressed by changes in nonverbal coordination. Patients with BPD display low levels of synchrony which are even below expected pseudosynchrony based on chance. The association between self‐reported childhood trauma and lower synchrony in BPD was most evident for patient’s imitative behaviour: Under inOT, patients with high scores of childhood trauma refrained from imitating their interview partners. Study limitations include small sample sizes and limited data on the psychological impact of the clinical interviews.