Background: Co-occurring mental health disorders are common among substance use disorder (SUD) patients. Medicaid expansion aimed to reduce barriers to SUD and mental health care and thereby improve treatment outcomes. Methods: We estimated change in the proportion of United States SUD treatment sites offering treatment for psychiatric comorbidities following Medicaid expansion as part of implementation of the Affordable Care Act (ACA) in 2014. Using panel data from the 2013–2014, n = 660, and 2016–2017, n = 638, waves of the National Drug Abuse Treatment System Survey (NDATSS), we estimated change in the proportion of sites offering antidepressant medication, other psychiatric medication, behavioral treatment, or any combination thereof for treatment of mental health comorbidities (i.e., beyond services focused on SUD). We modeled the impact of Medicaid expansion as an interaction between year and date of Medicaid expansion. We constructed a mixed-effects linear regression model for each outcome, with the interaction variable as the main exposure, site as a random effect, and site's average duration of treatment, proportion of clients with psychiatric comorbidities, average caseload per treatment prescribing-clinician on staff, type of facility and geographic region as covariates, to estimate a difference-in-differences (D-I-D) equation. Results: The adjusted D-I-D analysis indicated that the proportion of SUD treatment sites offering antidepressants for psychiatric treatment increased 10% (95% CI 1%, 18%) in the Medicaid expansion sites compared to non-expansion sites. The D-I-D for other psychiatric medications was also 10% (95% 1%, 19%). No significant changes were observed in behavioral treatment or the combination measure. The strongest association between Medicaid expansion and offering medication for mental health comorbidities was the 34% increase observed for residential treatment settings (95% CI 10%, 59%). Conclusion: Availability of psychiatric medication treatment in SUD treatment settings increased following Medicaid expansion, particularly in residential SUD facilities. This policy change has facilitated integrated treatment for the substantial share of SUD treatment patients with mental health comorbidities, with the greatest benefit for patients receiving SUD treatment in residential programs.