Background <p>Adverse childhood experiences (ACEs) include various forms of trauma or chronic stress during childhood, e.g., physical, emotional, and sexual abuse; neglect; and household dysfunction. ACEs have been linked to long-term psychological difficulties. Emerging research suggests that individuals with attention-deficit/hyperactivity disorder (ADHD) may be more frequently exposed to ACEs due to familial conflict or behavioral dysregulation. However, it remains unclear which specific ACE types mediate the association between ADHD symptoms and adult psychological distress.</p> Methods <p>We analyzed data from 29,268 adults in a nationwide Japanese internet-based survey. ADHD symptoms were measured using the Adult ADHD Self-Report Scale (ASRS-J-6), and severe psychological distress (SPD) was measured using the Kessler Psychological Distress Scale (K6) scale. Participants reported exposure to ten types of ACEs. Causal mediation analysis was performed to identify ACEs that mediated the ADHD–SPD association, adjusting for demographic variables.</p> Results <p>Among participants, 3.69% screened positive for ADHD symptoms, 12.81% met the criteria for SPD, and 5.09% reported four or more ACEs. Both ADHD symptoms and greater ACE exposure were significantly associated with SPD. Five ACE indicators, including emotional neglect, emotional abuse, physical abuse, household physical violence, and having four or more ACEs, significantly mediated the ADHD–SPD relationship. Emotional neglect had the strongest mediating effect (11.19%).</p> Conclusions <p>Emotion-related ACEs partially mediate the association between ADHD symptoms and adult psychological distress. While preventing childhood adversity remains essential, addressing ADHD symptoms through psychoeducation and intervention may reduce psychological burden in adulthood. These findings underscore the importance of trauma-informed approaches in the assessment and treatment of individuals with ADHD.</p> Clinical trial number <p>Not applicable.</p>