Background: Error processing is reflected, behaviorally, by slower reaction times (RT) on trials immediately following an error (post-error). Children with attention-deficit hyperactivity disorder (ADHD) fail to show RT slowing and demonstrate increased intra-subject variability (ISV) on post-error trials. The neural correlates of these behavioral deficits remain unclear. The dorsal anterior cingulate cortex (ACC) and lateral prefrontal cortex (PFC) are key regions implicated in error processing and subsequent behavioral adjustment. We hypothesized that children with ADHD, compared to typically developing (TD) controls, would exhibit reduced PFC activation during post-error (versus post-correct inhibition) trials and reduced dACC activation during error (versus correct inhibition) trials. Methods: Using functional Magnetic Resonance Imaging (fMRI) and a Go/No-Go task, we analyzed the neural correlates of error processing in 13 children with ADHD and 17 TD children. Results: Behaviorally, children with ADHD showed similar RT slowing but increased ISV compared to controls. The post-error contrast revealed a relative increase in blood-oxygen-level dependent (BOLD) signal in the middle/inferior temporal cortex (TempC), the ACC/supplementary motor area (SMA) and the somatosensory/auditory cortex (AudC) in children with ADHD compared to controls. Importantly, in the ADHD group, increased post-error temporal cortex activity was associated with lower ISV. During error (versus correct inhibition) trials, no between-group differences were detected. However, in children with ADHD lower ISV was associated with decreased insula and increased precentral gyrus activity. Conclusions: In children with ADHD, post-error neural activity suggests, first, a shift of attention towards task-irrelevant stimuli (AudC), and second, a recruitment of compensatory regions that resolve stimulus conflict (TempC) and improve response selection/execution (ACC/SMA). ADHD children with higher temporal cortex activation showed lower ISV, suggesting that functional abnormalities in the compensatory temporal regions contribute to increased variability. Moreover, increased ISV may be related to an over-sensitivity to negative outcomes during error trials in ADHD (insula correlation).