["Behavioral Sciences &the Law, EarlyView. ", "\nABSTRACT\nBullying perpetration and substance use are two prevalent and concerning forms of delinquent behavior worldwide. Although parent–child communication has been theorized to reduce adolescents' involvement in these behaviors, empirical evidence remains mixed. Teacher‐student relationship quality has been proposed as a potential mediator, but evidence from cross‐national studies is limited. Using data from the 2017/2018 Health Behavior in School‐Aged Children (HBSC) study, this study examined a theoretical model in which teacher–student relationship quality mediates the associations of parent–child communication with bullying perpetration and substance use. Potential sex differences in these interrelationships were also examined. Results indicated that parent–child communication was negatively associated with bullying perpetration and substance use, with teacher‐student relationship quality partially mediating these associations. Moreover, multi‐group analyses showed that the model fit well for boys and girls. These findings underscore the significance of fostering positive parent–child communication and teacher–student relationships to reduce adolescent bullying perpetration and substance use.\n"]