Background <p>Researchers are still learning about the far-reaching effects of the COVID-19 pandemic and its related societal responses on many aspects of life and behavior.</p> Objective <p>To analyze the pandemic’s impact on adolescent sexual behavior and substance use.</p> Methods <p>We use detailed longitudinal survey data on adolescent behavior including a pre-COVID comparison group and a pre-pandemic baseline for the COVID-affected group.</p> Results <p>During the pandemic, teens who were sexually experienced prior to the pandemic were 20 percentage point less likely to have had recent sex compared to prior cohorts from the same schools. Teens who were sexually inexperienced prior to the pandemic were 20 percentage points less likely to have begun having sex by fall 2020. We also find a decline in sexting relative to previous cohorts, especially among sexually experienced students. In contrast, we find that substance use among our sample did not change during the COVID-19 pandemic. Complicating this analysis, we find that the survey response rate declined significantly during the pandemic and this decline was larger for sexually experienced teens. To test for selective attrition, we examine whether plausible baseline correlates for subsequent sexual behavior predict survey response and whether this relationship becomes stronger during the COVID-19 pandemic. We also use a Heckman correction to adjust our analysis for selection. We find little evidence that the changes in sexual behavior were driven by selective attrition in the sample.</p> Conclusions <p>We find that sexual activity among young teens declined significantly during the COVID-19 pandemic.</p>