The use of negative campaigning is often explained by rational considerations; the more perceived benefits exceed perceived costs, the more likely candidates are to attack their political opponents. However, this benefit-cost ratio explains campaign behavior only to some extent. In this study, we test social norms as additional drivers of a candidate’s attack behavior. Social norms are informal rules that guide the behavior of members of a group. Descriptive norms—the perceived prevalence of a behavior—indicate an effective action. Injunctive norms—the perception of others’ approval of a behavior—indicate appropriate behavior. In a preregistered postelection survey among candidates of the six major parties in five German state elections (N = 1.087), we examine how perceived descriptive and injunctive norms of candidates of other parties, members of a candidate’s own party, and voters affect the decision to go negative in addition to the benefit-cost-ratio. The results show that the perceived descriptive norms among the members of a candidate’s own party (i.e., the perceived extent to which they use negative campaigning) and the perceived injunctive norms of a candidate’s voters (i.e., their perceived approval of negative campaigning) substantially affect a candidate’s attack behavior. No impact was observed for the perceived social norms of candidates from other parties.