Executive functions (EF) are impaired in children with attention-deficit/hyperactivity disorder (ADHD). It may be especially critical for interventions to target EF in early childhood given the developmental progression of EF deficits that may contribute to later functional impairments. This proof-of-concept study examined the initial efficacy of an intervention program on EF and ADHD. We also examined child performance on three neurocognitive tasks assessing cognitive flexibility, auditory/visual attention, and sustained/selective attention. Children with ADHD (ages 3-7) and their parents were randomized to receive an intervention targeting metacognitive EF deficits (n=13) or to a waitlist control condition (n=12). Linear model analysis of covariance compared groups on parent EF ratings, blinded clinician ratings of ADHD symptoms and improvement, and child performance on neurocognitive measures. Children who received the intervention significantly improved on parent ratings of attention shifting and emotion regulation in addition to clinician ratings of inattention. Moderate effect sizes showed additional intervention effects on parent ratings of inhibition, memory, and planning, and clinician ratings of hyperactivity/impulsivity and overall improvement. Small effect sizes were observed for improvement on child neurocognitive measures. Although replication with a larger sample and an active control group is needed, EF training with a metacognitive focus is a potentially promising intervention for young children with ADHD.