The COVID-19 pandemic had significant impacts on adolescents’ mental health and social interactions; however, little is known about cyber-victimization and mental health concerns from before to during the pandemic. The current study addressed this gap, while also examining how social media use and disagreements with friends during the pandemic were associated with cyber-victimization and mental health outcomes. Participants included 272 youth in the U.S. (56% female; 32% White), surveyed in fall 2019 ( M age = 11.75, SD = 0.68) and spring 2021 ( M age = 13.11, SD = 0.75). Adolescents reported increases in mental health symptoms and decreases in cyber-victimization. Experiencing more cyber-victimization before the pandemic was associated with significant increases in anxiety, depression, and social stress. The results suggest bolstering violence prevention programming in schools to reduce the likelihood of cyber-victimization and associated mental health outcomes.