This study examined the relationship between countries' dominant cultural values (i.e., individualism and collectivism) and (a) school principals' perceptions of aggressive student behavior and (b) students' self-reports of being aggressively victimized in school. Data on student aggression and victimization were collected across 62 countries in nationally representative samples of fourth and eighth graders (N = 428,566) and their principals (N = 15,043) by the Trends in International Mathematics and Science Study (TIMSS) 2007. Students were asked about three forms of aggressive victimization: physical, verbal, and relational; principals about two forms of aggressive student behavior: physical and verbal. Country-level regression analyses revealed that the level of cultural individualism, according to the individualism index (IDV) by Hofstede, Hofstede, and Minkov (2010), was not significantly related to either form of student-reported victimization. However, school principals reported aggressive student behavior more often the more individualist, and hence less collectivist, their country's culture. This relation was evident in the principals' reports on 4th and 8th grade students' aggressive behavior for both physical and verbal aggression. Multilevel analyses revealed that cultural individualism was still a powerful predictor of principal-reported aggressive student behavior after controlling for school and country characteristics. The discussion outlines reasons why principals' reports of aggressive student behavior are probably more valid indicators of student aggression than student self-reports of victimization, thereby supporting the hypothesis of culture-dependency of aggression.