Adolescents with Sluggish Cognitive Tempo (SCT) show symptoms of slowness, mental confusion, excessive daydreaming, low motivation, and drowsiness/sleepiness. Although many symptoms of SCT reflect internalizing states, no study has evaluated the utility of self-report of SCT in an ADHD sample. Further, it remains unclear whether SCT is best conceptualized as a unidimensional or multidimensional construct. In a sample of 262 adolescents comprehensively diagnosed with ADHD, the present study evaluated the dimensionality of a SCT scale and compared CFA and bifactor model fits for parent- and self-report versions. Analyses revealed the three-factor bifactor model to be the best fitting model. In addition, SCT factors predicted social and academic impairment and internalizing symptoms. Therefore, SCT as a multidimensional construct appears to have clinical utility in predicting impairment. Also, multiple reporters should be used, as they predicted different areas of functioning and were not invariant, suggesting that each rater adds unique information.