(1) Background: The purpose of the present study was to validate the Perseverative Thinking Questionnaire (PTQ) and the Buss–Perry Aggression Questionnaire-Short Form (BPAQ-SF) and test whether repetitive negative thinking plays an indirect role in the relationship between bullying victimization and aggression among Lebanese adolescents. (2) Methods: This cross-sectional study was conducted between January and May 2022 and included 379 Lebanese adolescent students (64.9% females, mean age 16.07 years). (3) Results: The three-factor solution of the PTQ and the four-factor solution of the BPAQ-SF showed excellent model fit. PTQ mediated the association between bullying victimization and physical aggression, verbal aggression, hostility, and anger. (4) Conclusions: This study expands on previous research by showing that repetitive negative thinking, an impactful socio-cognitive factor for students’ mental health, has a mediating (indirect) effect on the cross-sectional relationship between bullying victimization and aggression. This suggests that interventions aiming to prevent aggressive behaviors among adolescent students may be more effective if focused on repetitive negative thinking.