Synchronous interpersonal motor interactions require moment-to-moment predictions and proactive monitoring of the partner's actions. Neurophysiologically, this is highlighted by an enhancement of midfrontal theta activity. In the present study we explored the causal role of midfrontal theta for interpersonal motor interactions employing transcranial alternating current stimulation (tACS). We implemented a realistic human-avatar interaction task in immersive virtual reality (IVR) where participants controlled a virtual arm and hand to press a button synchronously with a virtual partner. Participants completed the task while receiving theta (Experiment 1) or beta (control frequency, Experiment 2) EEG-informed tACS over the frontal midline, as well as sham stimulation as a control. Results showed that frontal theta tACS significantly improved behavioural performance (by reducing interpersonal asynchrony) and participants' motor strategies (by increasing movement times and reducing reaction times), while beta tACS had no effect on these measures. These results suggest that theta tACS over frontal areas facilitates action monitoring and motor abilities supporting interpersonal interactions.