Sluggish cognitive tempo (SCT) was originally identified as a construct that characterized the inattention problems of some children with attention deficit disorder (ADD). Research has indicated that using SCT symptoms to identify a subset of youth with attention-deficit/hyperactivity disorder, predominately inattentive type (ADHD-IT) may elucidate distinct patterns of impairment and thereby improve the external validity of ADHD subtypes. The objective of the current study was to investigate whether youth with clinically-assessed ADHD-IT and high levels of SCT exhibit unique social and academic impairments. In a clinic-referred sample of youth (N = 209; 23 % female) aged 6 to 17 years, participants who met criteria for three different groups were identified: ADHD, Combined Type (ADHD-CT; n = 80), ADHD-IT with low SCT symptoms (n = 74), and ADHD-IT with high SCT symptoms (n = 55). These groups were compared on indicators of social and academic functioning while considering the effects of co-occurring internalizing and disruptive behavior disorders. Youth with ADHD-IT high in SCT exhibited uniquely elevated withdrawal, as well as low leadership and low peer-directed relational and overt aggression, which were not accounted for by co-occurring disorders. This high-SCT group was also the only group to have more homework problems than the ADHD-CT group, but only when other disruptive behavior disorders were absent. The distinctiveness of the high-SCT group, which was primarily evident in social as opposed to academic functioning, provides partial support for the external validity and clinical utility of SCT.