Justice-involved people with mental illnesses, in general, experience poor criminal justice outcomes (i.e., high rates of recidivism and probation revocations) and are at increased risk of homelessness, unemployment, stigma, trauma, and poor physical health. Low social support is repeatedly associated with worse mental health outcomes in the general population but little is known about social support among probationers with serious mental illnesses. Method: To address these gaps in the literature, we used an observational cross-sectional study design and data from a large, randomized controlled trial of specialty mental health probation to examine self-reported social support and its relationships with mental health functioning and other outcomes for individuals with serious mental illnesses on supervised probation. Results: Probationers who self-reported lower levels of social support also reported greater mental health symptomatology and reported lower quality relationships with their probation officers. Conclusions and Implications for Practice: Low social support among probationers with mental illnesses has important implications for mental health and criminal justice practice and policy. Coordinating services between the criminal justice and mental health systems to offer opportunities for social support and meaningful community engagement for those with mental illnesses who are on probation could improve a number of mental health and criminal justice outcomes for this population. Peer support and supported employment services, for example, in addition to outpatient mental health services, could be two strategies that could address social isolation and help individuals living with mental illnesses optimize their recovery and rehabilitation. Impact and ImplicationsAmong individuals with serious mental illnesses on probation, low levels of social support were associated with greater symptoms of depression and posttraumatic stress disorder, poor mental health functioning, and poor relationships with probation officers. Justice-involved individuals with mental illnesses would benefit from prosocial supports and meaningful community engagement opportunities to reduce social isolation, optimize recovery, and improve outcomes.