Aggression, an increasing problem among adolescents, is a potential threat to public health as it can lead to violence. Determining the factors causing aggression plays an important role in taking measures to reduce violence. This study aimed at determining the level of aggression among adolescents and at identifying the factors associated with high levels of aggression. This cross-sectional study was conducted with 2,409 Turkish adolescents. Data were collected with the Socio-demographic Questionnaire, Aggression Scale, Perceived Social Support Scale, and Communication Skills Attitude Scale. Data were analyzed using descriptive statistics, the chi-square test, t-test, and logistic regression. The participants’ mean aggression score was 91.83 ± 24.05, and 24.0% of the adolescents’ aggression levels rated high. According to the logistic regression model, aggression was 1.26 times higher among males, 1.92 times higher among those who perceived their mental health as poor, 1.58 times higher among those with suicidal ideation, 1.29 times higher among those who did not get prepared for university entrance exams, and 1.62 times higher among those who perceived their school performance as poor. Perceived family social support was a protective factor against high aggression. Approximately one out of every four adolescents in the two Turkish high schools where the study was conducted was determined to display high levels of aggression. Therefore, in order to reduce aggression among adolescents, programs such as coping management and coping with anger should be applied by nurses. Programs should include not only students but also families.