Cybervictimization has been linked to adverse psychological consequences but little is known about the mechanisms linking cybervictimization to lower well-being. We conducted two studies to examine emotional self-efficacy and distinct emotion regulation strategies as potential mediators in the relationship between cybervictimization and lower well-being among German adolescents during the school closures due to the COVID-19 pandemic in early 2020. In Study 1, 107 adolescents (
M
age
= 15.76) reported their cybervictimization frequency, emotional self-efficacy beliefs, and aspects of well-being (i.e., self-esteem, perceived social support, and subjective well-being during the COVID-19 related school closures). Emotional self-efficacy mediated the link between cybervictimization and all well-being measures. Specifically, cybervictimization was related to lower well-being through lower self-efficacy for managing negative emotions. For further examination, in Study 2, 205 adolescents (
M
age
= 15.45) were asked to report their cybervictimization experiences, use of specific emotion regulation strategies (rumination, reappraisal, and suppression), and well-being (i.e., self-esteem and life satisfaction). Cybervictimization was related to lower well-being through more rumination, but not through reappraisal or suppression. Taken together, our findings suggest that cybervictims may have lower emotional self-efficacy beliefs and engage in more rumination, a maladaptive emotion regulation strategy. These deficits in adolescents’ beliefs and capabilities for effectively managing negative emotions may be accountable for the adverse psychological consequences of cybervictimization. Notably, exploratory analyses suggest that cybervictimization frequency did not increase among adolescents during the lockdown (e.g., homeschooling, social distancing) due to the COVID-19 pandemic.