The purpose of the study was to evaluate the relation between Sluggish Cognitive Tempo (SCT) and academic functioning in a sample of 52 adolescents (40 males, 12 females) with Attention-Deficit/Hyperactivity Disorder (ADHD; M age = 13.75). This study builds on prior work by utilizing an empirically-based and psychometrically validated measure of SCT, collecting ratings of SCT from both parents and teachers, and examining associations with multiple domains of academic functioning from both the parent and teacher perspective as well as grade point average (GPA). Both SCT and DSM-IV symptoms of inattention were significantly correlated with domains of academic functioning. Multiple regression analyses demonstrated that the parent-rated SCT Slow subscale predicted overall academic functioning, organizational skills impairment, and homework problems above and beyond ADHD symptoms and child and demographic characteristics known to be associated with academics, including intelligence, academic achievement, and family income. The teacher-rated SCT Low Initiation/Persistence subscale also predicted homework problems and was the only SCT variable to predict school grades above and beyond ADHD symptoms and relevant covariates. Both the SCT Slow and Low Initiation/Persistence subscales include items related to youth seeming apathetic, unmotivated, and lacking initiative, behaviors that are strongly related to ADHD symptoms of inattention but not currently captured by the DSM-IV. Implications of these findings towards supporting the external validity of the SCT construct are discussed along with potential implications for intervention.