The Trends in International Mathematics and Science Study (TIMSS) provides internationally comparable data on students’ mathematics and science outcomes. The present study examines whether school bullying moderates gender differences in outcomes on the TIMSS mathematics test among fourth-grade students in Bosnia and Herzegovina. The sample consisted of 3,324 fourth-grade students (1,654 girls and 1,670 boys). Mathematics achievement was assessed using TIMSS plausible values, and analyses accounted for the complex survey design using the student sampling weights. Results showed a significant negative association between school bullying and mathematics achievement for both genders, as well as a significant gender × bullying interaction. Boys achieved higher scores at low and moderate levels of bullying, whereas girls outperformed boys under high bullying exposure. This pattern was consistent across all plausible mathematics values. The results suggest that efforts aimed at improving school social climate and reducing bullying may play an important role in promoting both students’ academic achievement and their well-being.