Background: Maternal stress levels and mental health symptoms have significantly increased since the coronavirus (COVID-19) pandemic began. As a result, experts have become concerned about elevated substance use given well-known associations between negative emotions and increased substance use. Extant research has not examined substance use among mothers during the COVID-19 pandemic which is a critical next step given links between maternal substance use and adverse childhood outcomes. Methods: Data was collected April 14th to 28th, 2020 from 508 mothers (Mage = 34.8 years, SD = 5.1) with young children between the ages of 0 to 8 years old via the Parenting During the Pandemic study. To identify associations between self-reported changes in substance use, mothers completed self-reported questionnaires on their substance use, motivations for using substances, and their mental health. Results: Of participating mothers, 54.9% did not change their substance use, 39.2% increased their use, and 5.9% decreased their use. Findings indicate that mothers with an anxiety disorder or clinically relevant anxiety symptomatology were more likely to report increased substance use during the COVID-19 pandemic. Using substances to cope with anxiety, but not depression or boredom, was significantly elevated among mothers who self-reported increased substance use during the pandemic relative to those reporting no change to or a decrease in usage. Using substances to cope with anxiety was a risk factor for increasing substance use. Discussion: A large proportion of mothers have reported increasing their substance use since the onset of the COVID-19 pandemic. Our results highlight the crucial need to treat maternal anxiety and the importance of teaching mothers alternative coping strategies, other than using substances, to reduce the incidence of increased substance use among mothers and its negative sequelae.