Importance Bullying is the most widespread form of peer aggression in schools. In an effort to address school bullying, 49 states have passed antibullying statutes. Despite the ubiquity of these policies, there has been limited empirical examination of their effectiveness in reducing students' risk of being bullied. Objective To evaluate the effectiveness of antibullying legislation in reducing students' risk of being bullied and cyberbullied, using data from 25 states in the United States. Design, Setting, and Participants A cross-sectional observational studywas conducted using a population-based survey of 63 635 adolescents in grades 9 to 12 from 25 states participating in the 2011 Youth Risk Behavior Surveillance System study (September 2010-December 2011). Data on antibullying legislation were obtained from the US Department of Education (DOE), which commissioned a systematic review of state laws in 2011. The report identified 16 key components that were divided into the following 4 broad categories: purpose and definition of the law, district policy development and review, school district policy components (eg, responsibilities for reporting bullying incidents), and additional components (eg, how policies are communicated). Policy variables from 25 states were linked to individual-level data from the Youth Risk Behavior Surveillance System on experiencing bullying and cyberbullying. Analyses were conducted between March 1, 2014, and December 1, 2014. Exposure State antibullying legislation. Main Outcomes and Measures Exposure to bullying and cyberbullying in the past 12 months. Results There was substantial variation in the rates of bullying and cyberbullying across states. After controlling for relevant state-level confounders, students in states with at least 1 DOE legislative component in the antibullying law had a 24%(95%CI, 15%-32%) reduced odds of reporting bullying and 20%(95%CI, 9%-29%) reduced odds of reporting cyberbullying compared with students in states whose laws had no DOE legislative components. Three individual components of antibullying legislation were consistently associated with decreased odds of exposure to both bullying and cyberbullying: statement of scope, description of prohibited behaviors, and requirements for school districts to develop and implement local policies. Conclusions and Relevance Antibullying policiesmay represent effective intervention strategies for reducing students' risk of being bullied and cyberbullied in schools.