Objective: Here we explored whether children with ADHD have a deficit in relational reasoning, a skill subtending the acquisition of many cognitive abilities and social rules. Method: We analyzed the performance of a group of children with ADHD during a transitive inference task, a task requiring first to learn the reciprocal relationship between adjacent items of a rank ordered series (e.g., A >B; B >C; C >D; D >E; E >F), and second, to deduct the relationship between novel pairs of items never matched during the learning (e.g., B >D; C >E). Results: As a main result, we observed that children with ADHD were impaired in performing inferential reasoning problems. The deficit in relational reasoning was found to be related to the difficulty in managing a unified representation of ordered items. Conclusion: The present finding documented a novel deficit in ADHD, contributing to improving the understanding of the disorder.