Objective: Few studies have examined whether separate dimensions of Sluggish Cognitive Tempo (SCT)—inconsistent alertness and slowness—have different external correlates from each other as well as symptoms of ADHD inattention (ADHD-IN). Method: Participants were 131 Spanish children (ages 6-16; 72% boys) diagnosed with ADHD. Results: In regression analyses, ADHD-IN was positively associated with hyperactivity/impulsivity, conduct problems, defiance/aggression, anxiety, peer relations problems, and learning problems. SCT-inconsistent alertness was positively associated with hyperactivity/impulsivity and peer relations problems. In contrast, SCT-slowness was negatively associated with hyperactivity/impulsivity and conduct problems and positively associated with depression and learning problems. Results were consistent after controlling for depression, medication status, and sex. Conclusion: The findings support SCT to be a construct with two dimensions that have unique correlates relative to each other as well as ADHD-IN. Future research on SCT should separate these dimensions of SCT to provide a better understanding of the construct.