question: IMPORTANCE: The COVID-19 pandemic and resulting conditions may negatively affect adolescents. OBJECTIVE: To examine aspects of self-reported mental and physical health among adolescents in Norway before and during the pandemic, including the role of pandemic-associated anxiety. DESIGN, SETTING, AND PARTICIPANTS: This cohort study examined a diverse nationwide sample of grade 11 students from the longitudinal MyLife study in Norway. The original study recruitment of all 8th, 9th, and 10th graders from the same middle schools facilitated identification of 2 sociodemographically comparable cohorts assessed in October to December 2018 and 2019, before the COVID-19 pandemic, and October to December 2020, during the pandemic. School entry and enrollment in Norway is determined by the birth year, and students usually start high school (11th grade) during the fall of the year of their 16th birthday. Data were analyzed from March to June 2021. EXPOSURES: The COVID-19 pandemic and associated conditions in Norway. MAIN OUTCOMES AND MEASURES: In grades 10 and 11, adolescents reported their depression symptoms using the Patient Health Questionnaire-9 (cutoff scores for moderate/severe depression, ≥15), number of close friends, physical health, and organized sports participation. Cohort differences were examined with a set of nested regression models, incrementally controlling for sociodemographic covariates and grade 10 outcomes. RESULTS: A sample of 2536 adolescents (1505 [59.4%] girls) was analyzed, including 1621 adolescents before the pandemic and 915 adolescents during the pandemic, of whom 158 adolescents (17.3%) reported high pandemic anxiety. The only significant difference in outcomes between the COVID-19 cohort and the pre–COVID-19 cohort were lower odds of organized sports participation (adjusted odds ratio [aOR], 0.69; 95% CI, 0.56-0.87). However, in subanalyses comparing adolescents with high anxiety during the COVID-19 pandemic with adolescents in the pre–COVID-19 cohort, adolescents with high pandemic anxiety were more likely to experience clinical-level depression symptoms (aOR, 2.17; 95% CI, 1.39-3.37) and poor physical health (aOR, 1.53; 95% CI, 1.01-2.31). CONCLUSIONS AND RELEVANCE: In this cohort study of Norwegian adolescents, adolescents who started high school during the pandemic year had lower odds of organized sports participation in late 2020, but were otherwise comparable in terms of self-reported mental and physical health with their pre–COVID-19 counterparts. However, adolescents in the COVID-19 cohort experiencing high pandemic-related anxiety had significantly greater odds of poorer mental and physical health than adolescents in the pre–COVID-19 cohort. Strategies aiming to mitigate the impact of COVID-19 may benefit from identifying youth disproportionally affected by the pandemic conditions.