Although there are likely to be multiple mechanisms underlying parent attention-deficit/hyperactivity disorder (ADHD) symptoms as a key risk factor for offspring ADHD, potential explanatory factors have yet to be reliably identified. Given that parent ADHD symptoms independently predict parenting behavior and child ADHD symptoms, we tested whether individual differences in multiple dimensions of positive and negative parenting behavior (i.e., corporal punishment, inconsistent discipline, positive parenting behavior, observed negative talk, and observed praise) mediated the association between parental and offspring ADHD. We used a prospective design that featured predictors (i.e., parent ADHD symptoms) and mediators (i.e., parenting behavior) that temporally preceded the outcome (i.e., offspring ADHD symptoms). Using a well-characterized sample of 120 children with and without ADHD (ages 5-10 at Wave 1, 7-12 at Wave 2) and their biological parents, we examined multimethod (i.e., observed, self-report) measures of positive and negative parenting behavior as simultaneous mediators of the association of Wave 1 parent and Wave 2 offspring ADHD symptoms. Using a multiple mediation framework, consisting of rigorous bootstrapping procedures and controlling for parent depression, child's baseline ADHD and oppositional defiant disorder, and child's age, corporal punishment significantly and uniquely mediated the association of Wave 1 parent ADHD symptoms and Wave 2 offspring ADHD. We consider the role of parenting behavior in the intergenerational transmission of ADHD as well as implications of these findings for the intervention and prevention of childhood ADHD.