Purpose: Reducing the spread of infectious disease during the coronavirus disease 2019 (COVID-19) pandemic prompted recommendations for individuals to limit physical contact with others and engage in social distancing. Less is known about why youth are engaging in social distancing and how these motivations are connected with their engagement, mental health, and social health. Using a large sample of adolescents from across the United States, this study examined adolescents’ motivations for social distancing, their engagement in social distancing, and their mental and social health. Methods: Data were collected March 29th and 30th 2020, two-weeks after COVID-19 was declared a national emergency in the United States. The sample consisted of 683 adolescents recruited using social media. A series of multiple linear regressions examined unique associations among adolescents’ motivations to engage in social distancing, social distancing behaviors, anxiety symptoms, depressive symptoms, burdensomeness, and belongingness. Results: The most commonly reported motivations for social distancing concerned social responsibility and not wanting others to get sick. Motivations concerning state or city lockdowns, parental rules, and social responsibility were associated with greater social distancing, whereas motivations concerning no alternatives were associated with less social distancing. Engagement in social distancing was not significantly associated with adolescents’ mental or social health. However, specific motivations for social distancing were differentially associated with adolescents’ anxiety symptoms, depressive symptoms, burdensomeness, and belongingness. Conclusions: Understanding adolescents’ motivations to engage in social distancing may help inform strategies to increase social distancing engagement, reduce pathogen transmission, and improve mental and social health during pandemics.