Purpose <p>This study aims to address existing mixed findings in the event-related potential error-monitoring (i.e., error-related negativity; ERN) literature in children with autism spectrum disorders (autism). This study is the first to examine the moderating roles of parent-reported attention-deficit/hyperactivity disorder (ADHD) symptoms (i.e., inattention/hyperactivityimpulsivity) on the relation between autistic traits and the ERN/ΔERN in children with and without autism.</p> Methods <p>Data was collected from 40 children (10—17&#xa0;years of age) diagnosed with (<i>n</i> = 17) and without (<i>n</i> = 23) autism matched on age and IQ. Participants completed a flanker task while ERPs were measured. Autistic traits were measured on the Autism Quotient, while ADHD symptoms of inattention and hyperactivity-impulsivity were assessed via parent report on the BASC-2/3 Attention Problems and Hyperactivity subscales. Four moderation models were conducted examining the separate effects of each ADHD symptom cluster on autistic traits predicting the ERN and ΔERN mean amplitudes.</p> Results <p>Results indicate that the autism group had higher reported Attention Problems and Hyperactivity scores, lower accuracy scores on a traditional Flanker task, and similar reaction times (RT) compared to neurotypically developing peers. Attention Problems moderated the relations between autistic traits and both the ERN and ΔERN, while Hyperactivity only moderated the relation between autistic traits and the ERN.</p> Conclusions <p>This study provides an important first step towards understanding how co-occurrence between autism and ADHD symptoms impact neurocognitive processes, such as error-monitoring. These findings suggest future works need to control for co-occurring ADHD symptomology when investigating ERPs in autism samples.</p>