Writing difficulties are common among children with attention-deficit/hyperactivity disorder (ADHD), but the nature of these difficulties has not been well studied. Here we relate behavioral, psycholinguistic, cognitive (memory/executive), and graphomotor measures to spelling skills in school-age girls with ADHD (n = 30) and an age-matched group of typically developed spellers (TYPSP, n = 35). When subdividing the ADHD group into those with poor (ADHDPSP, n = 19) and typical spelling (ADHDTYPSP, n = 11), the two subgroups did not differ with regard to inattentive or hyperactive-impulsive symptom severity according to parent or teacher ratings. Both ADHD subgroups also had equally severe difficulties in graphomotor control-handwriting and (parent ratings of) written expression as compared to the TYPSP group. In contrast, ADHDPSP had problems relative to ADHDTYPSP and TYPSP on phonological and orthographic recoding (choice tasks) and verbal memory (digit span) and were more likely to make commissions on a continuous performance task (CPT). Further analyses using the collapsed ADHD group showed that both digit span and the presence of CPT commissions predicted spelling performance independently of each other. Finally, results showed that phonological recoding skills mediated the association between digit span and spelling performance in ADHD. Theoretical and educational implications are discussed.