Although political radicalism is one of the major societal threats, we have limited understanding of how it is formed. While there are reasons to expect that harassment experienced in adolescence increase the propensity for radicalism, this relationship has not yet been investigated. This five-wave study of Swedish adolescents (N = 892) examined the role of peer harassment in radical political behavior. The results revealed that within-person fluctuations in harassment were positively related to fluctuations in radicalism. Individual-level (but not class-level) harassment also predicted differences between adolescents: youth who experienced more harassment had higher levels of and a more pronounced decrease in radicalism. In addition, adolescents who had more supportive teachers or parents were less affected by harassment than youth with less-supportive adults. The findings suggest that personal experiences of harassment increase the risk of radicalism but supportive relationships can mitigate their negative consequences.