Objective/Background This study aimed to examine the relationship between sleep problems and working memory in children aged 5–13 years with attention-deficit/hyperactivity disorder (ADHD). Patients/Methods Children with ADHD were recruited into a randomized controlled trial from 21 paediatric practices in VIC, Australia. Cross-sectional data for intervention and control children were pooled at 6 months post randomization for the current analyses (n = 189). Children who met the Diagnostic Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) criteria for ADHD and had a parent-reported moderate/severe sleep problem that fulfilled diagnostic criteria for a behavioural sleep disorder were recruited into the study. Sleep was assessed by detailed parent (Children's Sleep Habits Questionnaire) and self-reports (Self-Sleep Report). Working memory was measured using the Working Memory Test Battery for Children (low and very low working memory defined as <25th and <10th percentiles, respectively). Analyses were adjusted for child age and gender, internalizing and externalizing comorbidities, and socio-economic status. Results Self-reported sleep problem severity was associated with poorer working memory; for each standard deviation increase in self-reported sleep problems, working memory scores decreased by −3.8 points (95% confidence interval (CI): −6.7, −0.8; p = 0.01). There was some evidence that self-reported sleep problems were associated with low (p = 0.06) and very low working memory (p = 0.01). There was minimal evidence that parent-reported sleep problems were associated with poorer working memory with the exception of bedtime resistance problems. Conclusions Behavioural sleep problems and working memory are associated in children with ADHD, particularly when sleep is assessed by self-report.