Although parent-adolescent and peer-adolescent relationship quality are critical for adolescent wellbeing during typical stressful life events, the unique features of the COVID-19 pandemic put into question whether strong parent-adolescent and peer-adolescent relationship quality functioned as protective factors of adolescent mental health in this context. The current longitudinal study examined a community sample of adolescents across 3 time points, each 6 months apart (Time 1: Fall, 2019; n = 163, 50.9% male; mean age = 15.75 years, SD = 1.02). Results showed that increases in depression symptoms, perceived stress, and emotion dysregulation from Fall 2019 to Fall 2020 were predicted by changes in parent, but not peer relationship quality. The current study demonstrates that adolescent-parent relationship quality may be protective against mental health difficulties during the COVID-19 pandemic, while adolescent-peer relationship quality may not. Identifying protective factors that may play a role in mitigating the impact of the pandemic, and other such widespread health crises, on youth mental health is critical in reducing the long-term psychological harm of the viral outbreak, as well as promoting adolescent wellbeing and resilience.