Aim: Autism Spectrum Disorder (ASD) involves complex alterations in brain structure that persist across the lifespan. While structural brain alterations are known in children, the persistence of these neuroanatomical differences into adulthood remains less understood. This study examines the neuroanatomical basis of ASD in adulthood, specifically investigating how cortical thickness (CT) and structural similarity networks (SSN) are organized within the social brain network. Materials and Methods: T1-weighted MRI data were obtained for 24 adults with ASD and 24 neurotypical (NT) controls (ages 18–30) from the OpenNeuro dataset (ds002522). Image preprocessing was performed using the recon-all pipeline in FreeSurfer. We investigated CT and SSN at both: (1) the whole-brain, and (2) a hypothesis-driven level targeting 14 specific social brain network regions. CT was assessed using vertex-wise surface-based morphometry, while SSN were constructed using the Morphometric INverse Divergence (MIND) method. MIND quantifies morphological similarities based on the divergence of regional distributions for thickness, volume, surface area, mean curvature, and sulcal depth. Results: The SSN analysis revealed significantly increased nodal connectivity strength in the ASD group within the right posterior insula (pFDR=0.04) and the orbital part of the right inferior frontal gyrus (pFDR = 0.04). ROI-based CT comparisons and whole-brain SSN analyses showed no significant group differences. Conclusion: Our findings reveal a neuroanatomical signature in adults with ASD, characterized by localized structural hyper-connectivity within the inferior frontal gyrus and the insula. These results highlight that adult ASD is defined by persistent structural anomalies, manifesting as atypically high structural similarity within key social brain nodes rather than widespread, global network disruptions.