Purpose The purpose of this study was to compare pre/post-COVID-19 changes in mental health–related emergency department visits among adolescents. Methods We conducted a retrospective analysis of all mental health–related emergency department visits in two large tertiary pediatric hospital centers between January 2018 and December 2020. We described monthly pre/post-COVID-19 changes in frequency and proportion of mental health visits as well as changes in hospitalization rates for eating disorders, suicidality, substance use, and other mental health conditions. Results We found an increase in the proportion of mental health–related emergency department visits during the months of July–December 2020 (p < .01). There was a 62% increase in eating disorder visits between 2018–2019 and 2020 (p < .01). No pre pandemic/postpandemic changes were found in the proportion of visits resulting in hospitalization for any of the four diagnostic categories. Conclusions Our study suggests significant impacts of COVID-19 on adolescent mental health and a need for further longitudinal research work in this area.