This study investigates the association between polyunsaturated fatty acid (PUFA) intake and neurocognitive functions in children with attention-deficit/hyperactivity disorder (ADHD). We recruited 21 drug-naïve children diagnosed with ADHD according to the Diagnostic and Statistical Manual of Mental Disorders and 21 non-ADHD controls. The n-3 intake and essential fatty acid (EFA) deficiency severity were recorded while the children were assessed for inhibitory control, delay aversion, and temporal processing with the Go/No Go Task, Delayed Reaction Time Task, and Finger Tapping Task, respectively. The ADHD group had more EFA deficiency symptoms (p = .02) and poorer performance in delay aversion (p = .02) and temporal processing (p < .001). Moreover, ADHD symptoms correlated negatively with n-3 intake and positively with EFA deficiency. In addition, EFA deficiency was associated with higher delay aversion (p < .001). Children with ADHD had a higher deficiency of EFA, and EFA deficiency had a positive association with ADHD severity and delay aversion.