Background: Neurological subtle signs (NSS) are minor neurological abnormalities that have been shown to be increased in a number of neurodevelopmental conditions. For attention deficit/hyperactivity disorder (ADHD), it remains unclear whether NSS may aid the clinical diagnostic process. Methods: This study explored the association of total and specific domains of NSS in 1,055 children and adolescents with ADHD compared to 130 age-matched typically developing participants; the relationship between NSS and Spatial Working Memory (SWM) as a cognitive process integrally involved in ADHD was also assessed. To determine the diagnostic and predictive efficiency of NSS, a receiver operating curve analysis was performed and the area under the curve (AUC) quantified. The best discriminant points for differentiating between ADHD and typically developing participants and the predictive power of NSS for SWM impairment in ADHD young people were also calculated. Results: Area under the curves for total NSS, smoothness/accuracy, cerebellar signs and choreo-athetoid movements scores were considered good (0.84, 0.79, 0.74 and 0.73 respectively), and the results remained after controlling for gender and IQ. A total score of 13 or over on the Scored Developmental Neurological Examination proved to be a good threshold point for differentiating between the ADHD and typically developing participants. For ADHD children, the AUC of total NSS to distinguish between those below 25th and above 75th percentile were 0.77 and 0.73 for Spatial Span and for Between Search Errors respectively (the two SWM-dependent measures examined). Conclusions: This study provides evidence suggesting that NSS may aid the clinical evaluation of a child or adolescent with ADHD. In children and adolescents with ADHD, NSS are associated with difficulties in SWM, specifically the Spatial Span and Between Search Error components.