The COVID-19 pandemic has potentially increased the risk for adolescent depression. Even pre-pandemic, <50% of youth with depression accessed care, highlighting needs for accessible interventions. Accordingly, this randomized controlled trial (ClinicalTrials.gov: NCT04634903 ) tested online single-session interventions (SSIs) during COVID-19 in adolescents with elevated depression symptoms (N = 2,452, ages 13-16). Adolescents from all 50 US states, recruited via social media, were randomized to one of three SSIs: a behavioural activation SSI, an SSI teaching that traits are malleable and a supportive control. We tested each SSI's effects on post-intervention outcomes (hopelessness and agency) and three-month outcomes (depression, hopelessness, agency, generalized anxiety, COVID-19-related trauma and restrictive eating). Compared with the control, both active SSIs reduced three-month depressive symptoms (Cohen's d = 0.18), decreased post-intervention and three-month hopelessness (d = 0.16-0.28), increased post-intervention agency (d = 0.15-0.31) and reduced three-month restrictive eating (d = 0.12-17). Several differences between active SSIs emerged. These results confirm the utility of free-of-charge, online SSIs for high-symptom adolescents, even in the high-stress COVID-19 context.