Background: Exploring neural network dynamics during social interaction could help to identify biomarkers of Autism Spectrum Disorders (ASD). Recently, the cerebellum, a brain structure that plays a key role in social cognition, has attracted growing interest. Here, we investigated the electrophysiological activity of the cortico-cerebrum network during real-time social interaction in ASD. We focused our analysis on theta oscillations (3-8 Hz), which have been associated with large-scale coordination of distant brain areas and might contribute to interoception, motor control, and social event anticipation, all skills known to be altered in ASD. Methods: We combined the Human Dynamic Clamp, a paradigm for studying realistic social interactions using a virtual avatar, with high-density electroencephalography (HD-EEG). Using source reconstruction, we investigated power in the cortex and the cerebellum, along with coherence between the cerebellum and three cortical areas, and compared our findings in a sample of participants with ASD and with typical development (TD) (n = 140). We developed an open-source pipeline to analyse neural dynamics at the source level from HD-EEG data. Results: Individuals with ASD showed a significant increase in theta band power during social interaction compared to resting state, unlike individuals with TD. In particular, we observed a higher theta power over the cerebellum and the frontal and temporal cortices in the ASD group compared to the TD group, alongside bilateral connectivity alterations between the cerebellum and the sensorimotor and parietal cortices. Conclusions: This study uncovered ASD-specific alterations in the theta dynamics, especially in a network between the cerebellum and social-associated cortical networks.