ImportanceAdolescence is a period in which mental health problems emerge. Research suggests that the COVID-19 lockdown may have worsened emotional and behavioral health.ObjectiveTo examine whether socioeconomic status was associated with mental health outcomes among youths during the COVID-19 pandemic.Design, Setting, and ParticipantsThe Adolescent Brain Cognitive Development (ABCD) Study is a multisite 10-year longitudinal study of youth neurocognitive development in the US. Recruitment was staggered where the baseline visit (ages 9 to 10 years) occurred from 2016 to 2018, and visits occurred yearly. The COVID-19 lockdown halted research collection during the 2-year follow-up visits (ages 11 to 12 years), but eventually resumed. As some youths already underwent their 2-year visits prior to lockdown, this allowed for a natural experiment-like design to compare prepandemic and intrapandemic groups. Thus, data were gathered from the 1-year follow-up (pre–COVID-19 lockdown for all youths) and the 2-year follow-up, of which a portion of youths had data collected after the lockdown began, to compare whether a period of near social isolation was associated with mental health symptoms in youths. The prepandemic group consisted of youths with a 2-year follow-up visit collected prior to March 11, 2020, and the intrapandemic group had their 2-year follow-up visit after lockdown restrictions were lifted.Main Outcomes and MeasuresAssessments included measures on income-to-needs ratio (INR; derived from total household income), the Child Behavior Checklist (a measure of mental health symptomology), and the Family Environmental Scale.ResultsThe final sample included 10 399 youths; 3947 (52.3%) were male; 2084 (20.3%) were Latinx/Hispanic; 6765 (66.0%) were White; 4600 (44.2%) reported caregiver education levels below a 4-year college degree; and 2475 (26.2%) had INR either below 100% (indicating poverty) or between 100% and less than 200% (near poverty). Among youths in the intrapandemic group, worse mental health symptoms (eg, more total problems, greater depression, and greater anxiety) over time were associated with being from a household with higher socioeconomic status (eg, when comparing individuals who differed by 1 unit on INR between prepandemic and intrapandemic groups from 1-year to 2-year follow-up, their expected difference in total problems score was 0.79 [95% CI, 0.37-1.22]; false discovery rate–corrected P < .001).Conclusions and RelevanceThis cohort study found that the COVID-19 lockdown was associated with disproportionately negative mental health outcomes among youths from higher socioeconomic status backgrounds. Although this study does not shed light on the direct mechanisms driving these associations, it does provide some support for positive outcomes for youths. Future studies are needed to understand whether these associations persist over longer periods of time.