Theory and research suggest that parents’ reactions to children’s emotions play a critical role in teaching children effective emotion regulation (ER) skills, but no studies have directly examined the role that parent emotion socialization plays in the development of ER in children with ADHD. Gaining insight into the causes of impaired ER, particularly in youth with ADHD who are known to have poor ER, has important theoretical and translational significance. The present study is the first to longitudinally examine whether emotion socialization predicts later physiological and adult-reported measures of ER in children with and without ADHD. It also sought to determine if these relations are moderated by ADHD symptoms. Participants were 61 children (31 girls, 30 boys; M = 10.67 years, SD = 1.28) with and without clinically significant ADHD symptoms. At Time 1, parent reports of emotion socialization and parent- and teacher-report of child ADHD symptoms were collected. At Time 2, child ER measures were collected based on parent- and teacher-report and physiological reactivity during an impossible puzzle and a social rejection task. Physiological measures included respiratory sinus arrhythmia and skin conductance level (SCL). Supportive parenting practices were associated with better parent-rated emotion regulation skills for all children and greater SCL reactivity for children with high ADHD symptoms. Non-supportive parenting reactions were associated with greater adult-rated emotional lability for children with high ADHD symptoms. Results highlight the importance of considering multiple aspects of ER, including physiological manifestations. Findings suggest that parents’ use of adaptive emotion socialization practices may serve as a protective factor for children’s ER development and may be particularly critical for youth with ADHD. Our findings support the use of interventions addressing parent emotion socialization to help foster better ER in children.