Immigrant youth are at risk of experiencing harassment in school; however, we have only limited understanding of what makes youth harass their peers on ground of their ethnic origin. To address this major limitation, we examined (a) whether youth's negative attitudes toward immigrants impact their engagement in ethnic harassment over time and (b) whether youth's impulsivity, their tendencies to engage in risky behaviors, and a chaotic surrounding school environment moderate the link between their negative attitudes toward immigrants and their involvement in ethnic harassment. The sample included 583 Swedish youth (X-bar[subscript age] = 13.93, SD = 0.71). Youth with negative attitudes toward immigrants ethnically were found to harass their immigrant peers when they had high levels of impulsivity and violent tendencies. Contrary to our expectation, youth perceptions of school atmosphere did not act as a moderator. The present study highlights the importance of identifying risk factors to reach a comprehensive understanding of ethnic harassment.