Purpose: To assess changes in adolescent visits with substance use disorders (SUDs) at children's hospitals during COVID-19. Methods: We conducted a retrospective cohort study of adolescents (11–18 years) with SUD diagnoses during a hospital visit in the Pediatric Health Information System. Study periods were defined as spring (03/15–05/31), summer (06/01–08/31), and fall (09/01–12/31), pre-COVID-19 (2017–2019), or during COVID-19 (2020). We summarized the change in weekly median visits and interquartile range (IQR) by characteristics and compared between the two periods (e.g., spring pre-COVID-19 [3/15–05/31/2017–2019] versus spring COVID-19 [3/15–05/31/2020]) using median regression. Results: There were 42,979 SUD visits (10,697 COVID-19; 32,282 pre-COVID-19) and 256 annual weekly median adolescent SUD visits [IQR 235, 280] pre-COVID-19 and 268 [IQR 245, 278] during COVID-19. The median number of weekly SUD visits increased by 14.3% during summer COVID-19 (median visits, [IQR]: 272 [268, 278]) compared to pre-COVID-19 (median visits: 237, IQR [216, 249]; p < .001) but did not significantly differ in spring (p = .091) or fall (p = .65) COVID-19. Discussion: Our findings suggest increased problematic adolescent substance use during summer COVID-19. Efforts to increase the identification and treatment of adolescent SUDs remain critical.