The Social Brain is a distributed network of neuroanatomical regions and neurochemical systems that underpins the human capacity for social cognition, empathy, and interpersonal behavior. Social isolation (SI), defined as the objective reduction in social interaction, poses a significant threat to the integrity of this system. In this review, we synthesize evidence from human and animal studies to elucidate the biological, cognitive, and behavioral consequences of SI on the social brain. We describe how SI acts as a chronic stressor, disrupting structural connectivity, and altering neurotransmitter systems critical for social cognition. These disruptions manifest in altered social behavior, mentalization processes, and emotional reactivity, significantly contributing to increased vulnerability to psychiatric and neurodegenerative disorders, including depression, schizophrenia, substance use disorders, and Alzheimer’s disease. Converging findings from studies of evolutionarily conserved mechanisms in rodent and primate models demonstrate that SI compromises neurodevelopment, attenuates neuroplasticity, and triggers maladaptive stress responses, highlighting that social deprivation has profound neurobiological and behavioral consequences that greatly overlap with the pathophysiological changes seen in neuropsychiatric disorders. Furthermore, we explore the role of indirect stressors resulting from SI such as touch deprivation and digital-era social disconnection as contemporary amplifiers of SI’s neurobiological impact. In light of public health challenges such as the COVID-19 pandemic, we propose that SI should be recognized not only as a psychosocial condition but as a modifiable risk factor with transdiagnostic significance across psychiatry, neurology, and preventive medicine. Addressing SI through targeted interventions and policy measures is essential for promoting mental resilience and well-being.