ABSTRACT Homelessness and serious mental illness (SMI) are persistent and interrelated public health concerns in the United States, often associated with fragmented care and recurrent psychiatric hospitalization. However, limited research has examined how homelessness is associated with mental health treatment utilization among adults with SMI. This study used state-level administrative data from the Substance Abuse and Mental Health Services Administration (SAMHSA), including the Mental Health Client-Level Data, to assess associations between homelessness and the utilization of different types of mental healthcare services among adults with SMI. Community mental health system treatment utilization was operationalized as a series of binary measures indicating whether an individual received a specific category of service (e.g., SMHA-funded community-based services, state psychiatric hospital services, residential treatment center services, justice system services, or other inpatient psychiatric services). Descriptive statistics, bivariate correlations, and multivariable regression models, adjusting for sociodemographic characteristics, were employed to examine these associations. Findings suggest that there was a negative association between homelessness and community mental health system treatment utilization; however, there was a positive association between homelessness and the utilization of state psychiatric hospital, residential treatment center, justice system, and other psychiatric inpatient services utilization. These results suggest that individuals who are homeless may be less likely to use community mental health services, and more likely to use state psychiatric hospital, residential treatment center, justice system, and other psychiatric inpatient services. Strengthening community-based mental health infrastructure and supportive housing systems may be critical for improving housing instability.