Background/Objective: Our research examines changes in admissions to substance use disorder (SUD) treatment facilities associated with the COVID-19 pandemic. Methods: This observational study used data from the California Outcomes Monitoring System (CALOMS) to examine changes in California in admissions to SUD treatment facilities from January 1, 2019, to October 31, 2020 (before and during the COVID-19 pandemic). To align with California's COVID-19 response timeline, we identified January 1, 2019, through February 29, 2020, as pre-COVID-19 months, and March 1, 2020, to October 31, 2020, as post-COVID-19 months. We then plotted the average monthly admissions over the pre- and post-COVID-19 period. We calculated the statistical significance of the percent change in admissions pre- and post-COVID-19 using linear regression analysis. We also assessed how admission rates changed within demographic and treatment type subpopulations by calculating the mean monthly admission rates for the pre- and post-COVID-19 periods and the percent change between periods. Results: Average monthly admissions were 28% lower in the COVID-19 period (March through October 2020; mean=8,994) than in the pre-period (January 2019 through February 2020; mean=12,544). There was an equivalent decline in outpatient and residential admissions. The largest decline (38% or greater) occurred among admissions of individuals younger than 25, those referred from driving under the influence programs or drug court, those reporting a prison stay in the last 30 days, and individuals indicating that marijuana was their primary drug of abuse. Conclusions: These results suggest that COVID-19 was associated with a decline in specialty treatment for substance use disorders in California, despite increased use of telemedicine and other mitigation efforts. Rising rates of substance use overdose during the pandemic and declining use of specialty treatment highlight the need for better disaster planning to ensure access to substance use disorder treatment is maintained during emergencies.