Background:Since April 2020, some high schools were closed to in-person teaching, and interscholastic and club sports were cancelled in an attempt to slow the spread of COVID-19. While some US adolescents participated in sports since April 2020, other US adolescents did not participate in sports during this time. It is unknown what effect sport participation has had on the health of adolescents.Hypothesis/Purpose:To identify how sport participation during the COVID-19 pandemic impacted the health of adolescents.Methods:Adolescents across the US were recruited via social media to complete an online survey in the spring of 2021. Participants were asked to report their demographics (age, gender, race), whether they participated in school club or school sports since May 2020, type of school they attended (in-person, online, hybrid), and measures of mental health (MH), physical activity (PA) and quality of life (QoL). Assessments included the: General Anxiety Disorder-7 Item (GAD-7) for anxiety, Patient Health Questionnaire-9 Item (PHQ-9) for depression, Pediatric Functional Activity Brief Scale (PFABS) for physical activity, and Pediatric Quality of Life Inventory 4.0 (PedsQL) for quality of life. Assessment scores were compared for respondents who participated in sports (PAR) with those that did not participate in sports (NoPAR). Univariable comparisons between the groups were made via t-tests or chi-square tests while means for continuous outcome measures were compared between the groups by ANOVA models that controlled for age, gender, race and the type of school attendance.Results:4,693 adolescents (52% female, Age = 16.1+1.3 yrs., grades 9–12) from 38 states participated in the study with PAR = 4,286 (91%) and NoPAR = 407 (9%). NoPAR participants reported a higher prevalence of moderate to severe levels of anxiety (29.3% vs 21.3%, p<0.001) and depression (40.7% vs 19.8%, p<0.001). NoPAR participants reported lower (worse) PFABS scores (mean 13.2 [95%CI 12.4, 13.9] vs 21.2 [20.7, 21.6] p<0.001) and lower (worse) PedsQL total scores compared to the PAR group (77.9 [76.5, 79.3] vs. 83.2 [82.3, 84.1], p<0.001).Conclusions:Adolescents who did not play a sport during the COVID-19 pandemic reported significantly higher levels of anxiety and depression, as well as lower physical activity and quality of life scores compared to adolescents who did play a school or club sport. Participation in organized sports may offer an important opportunity to improve physical activity and mental health for adolescents during and after the COVID-19 pandemic.