The role of oxytocin in social cognition has attracted tremendous interest in social neuroscience and psychiatry. Some studies have reported improvement in social symptoms following oxytocin treatment in autism spectrum disorders (ASD), while others point to endogenous factors influencing its efficiency and to mixed results in terms of long-term clinical benefits. Epigenetic modification to the oxytocin receptor gene (OXTR) in ASD could be an informative biomarker of treatment efficacy. Yet, little is known about the relationship between OXTR methylation, clinical severity, and brain function in ASD. Here, we investigated the relationship between OXTR methylation, ASD diagnosis (in N = 35 ASD and N = 64 neurotypical group), measures of social responsiveness, and resting-state functional connectivity (rsFC) between areas involved in social cognition and reward processing (in a subset of ASD, N = 30). Adults with ASD showed higher OXTR methylation levels in the intron 1 area compared with neurotypical subjects. This hypermethylation was related to clinical symptoms and to a hypoconnectivity between cortico-cortical areas involved in theory of mind. Methylation at a CpG site in the exon 1 area was positively related to social responsiveness deficits in ASD and to a hyperconnectivity between striatal and cortical brain areas. Taken together, these findings provide initial evidence for OXTR hypermethylation in the intron area as a potential biomarker for adults with ASD with less severe developmental communication deficits, but with impairments in theory of mind and self-awareness. Also, OXTR methylation in the exon 1 area could be a potential biomarker of sociability sensitive to life experiences.