Objective: This study evaluated whether sluggish cognitive tempo (SCT) is separable from ADHD–inattention (IN) and uniquely associated with internalizing dimensions in preschool children in South Korea. Method: Mothers of 172 preschool children (ages 4-6 years; 52% girls) rated children’s SCT, ADHD-IN, ADHD–hyperactivity/impulsivity (HI), oppositional defiant disorder (ODD), aggression, emotional reactivity, anxiety/depression, somatic complaints, withdrawal, and sleep problems. Results: Eight of 10 SCT symptoms showed convergent and discriminant validity with ADHD-IN. ADHD-IN remained significantly positively associated with ADHD-HI, ODD, and aggressive behavior after controlling for SCT, whereas SCT was no longer positively associated with these externalizing behaviors after controlling for ADHD-IN. Both SCT and ADHD-IN were uniquely associated with greater emotionally reactivity, anxiety/depression, and withdrawal. Only SCT was uniquely associated with somatic complaints, and only ADHD-IN was uniquely associated with sleep problems. Conclusion: Findings replicate results with children and adolescents, thus expanding evidence for the validity of SCT in early development.