Objectives Attention‐deficit/hyperactivity disorder (ADHD) is characterized by inattention, hyperactivity, and impulsivity. However, this triad might not be able to explain the complete spectrum of ADHD symptoms, as emotional dysregulation (ED) frequently seems to accompany the disorder. The aim of this study was to further understand the role of ED in adult ADHD. Method The sample comprised 393 adults with ADHD without or with comorbidity, and 121 adults without ADHD or any other mental disorder. Additionally, the sample focused on ED. The contribution of core symptoms and the effect of comorbidity on ED were tested and the predictive value of ED for the ADHD diagnosis itself analyzed. Finally, all subjects were categorized into groups–No ADHD, ADHD, and ADHD + ED–to analyze the differences in the severity of ADHD symptomatology in the three groups. Results ED levels were found to be elevated in patients with ADHD. The core symptoms affected ED, and the ADHD diagnosis was predicted by ED. The addition of ED to a regression model with the core symptoms was shown to improve the predictability of the ADHD diagnosis. The presence of ED proved to be an indicator of the severity of adult ADHD independent of a present comorbidity. Conclusions ED is a significant symptom in adult patients with ADHD and appears to be associated with ADHD itself. Whilst the presence of other mental disorders intensifies symptoms of ED, ED seems not to manifest solely as a consequence of comorbidity.