Background: The aim of this paper was to examine the early impact of COVID-19 on substance use to assess implications for planning substance use treatment and support systems. Method: A systematic review of literature published up to March 2021 was conducted to summarize changes in prevalence, incidence, and severity of substance use associated with COVID-19 and the accompanying public health measures, including lockdown, stay-at-home orders, and social distancing. Results: We identified 53 papers describing changes to substance use at the population level. The majority of papers described changes related to alcohol use and most relied on self-reported measures of consumption during the COVID-19 pandemic, compared with pre-pandemic use. There was less evidence to support changes in non-alcohol substance use. In general, risky pre-pandemic alcohol use, caregiving responsibilities, stress, depression, anxiety, and current treatment for a mental disorder were found to be associated with increased substance use. Conclusion: This review provides preliminary data on changes in substance use, indicating that certain segments of the population increased their alcohol use early on in the COVID-19 pandemic and may be at greater risk of harm and in need of additional services. There is a need for additional population-level information on substance use to inform evidence-based rapid responses from a treatment system perspective.