Although parent reactions to children’s negative emotions are important to the development of adolescent social and emotional functioning, there is a lack of research examining this aspect of parenting in samples that include youth with attention-deficit/hyperactivity disorder (ADHD). This study addresses this gap in the research by examining the independent effects of childhood ADHD symptoms and parent reactions to negative emotions in the longitudinal prediction of adolescent emotion dysregulation and peer problems. A sample of 124 youth (52% female) with and without clinical elevations in ADHD symptoms were assessed in childhood (8–12 years; M = 10.50) and followed up 5–6 years later in adolescence (13–18 years; M = 16.15). Path models tested the direct effects of childhood ADHD symptoms, supportive parent reactions, and non-supportive parent reactions on adolescent peer problems (friendship quality, deviant peer affiliation, peer aggression) and the indirect effects via adolescent emotion dysregulation. Emotion dysregulation mediated the effects of greater ADHD symptoms and of less parent supportive reactions on adolescent peer problems; parent reactions also independently predicted specific adolescent peer problems. Even for youth with clinical elevations in ADHD symptoms, parent reactions to children’s negative emotions may be important in understanding adolescent emotion dysregulation and peer problems.