Synchronous interpersonal motor interactions require moment-to-moment prediction and proactive monitoring of the partner's actions. Neurophysiologically, this is highlighted by an enhancement of midfrontal theta (4–7 Hz) oscillations. In this study, we explored the causal role of midfrontal theta for interpersonal motor interactions using transcranial alternating current stimulation (tACS). We implemented a realistic human–avatar interaction task in immersive virtual reality where participants controlled a virtual arm and hand to press a button synchronously with a virtual partner. Participants completed the task while receiving EEG-informed theta (Experiment 1) or beta (control frequency, Experiment 2) tACS over the frontal midline, as well as sham stimulation as a control. Results showed that midfrontal theta tACS significantly improved behavioral performance (i.e., reduced interpersonal asynchrony) and participants' motor strategies (i.e., increased movement times and reduced RTs), whereas 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.