In this study, our aim was to evaluate the internal and external validity of Sluggish Cognitive Tempo (SCT) and to determine if it is an independent factor from Attention Deficit Hyperactivity Disorder (ADHD) in Turkish children, like in other cultures. Two hundred sixty-one children (6–12 years of age) who applied to Ankara University Child and Adolescent Psychiatry outpatient clinics and diagnosed with ADHD recruited the study. All children were evaluated with the Schedule for Affective Disorders and Schizophrenia for School-Age Children Present and Lifetime Version which is a semi-structured diagnostic interview (K-SADS-PL) for ADHD diagnosis. Child Behavior Checklist for ages 6–18, The Barkley’s Child SCT Ratings Scale, SNAP-IV Parent and Teacher Scale, and Sociodemographic Information Form were enrolled by the parents and teachers of the children. Our results demonstrated that SCT symptoms formed two distinct but interrelated factors (Sluggish and Daydreaming) separate from those for ADHD. Due to regression analyses, higher levels of SCT predicted higher levels of ADHD-IN (Inattentive) and internalizing symptoms including anxiety-depression, social problems, and social withdrawal. These findings distinguished SCT cases from ADHD-IN in Turkish children. Results also indicated that ADHD- IN symptoms are risk factors for lower academic achievement while SCT symptoms haven’t such an effect. Our study is the first which demonstrates SCT’s construct validity relative to ADHD-IN by parental and teacher ratings in Turkey. Similar findings with Turkish children to the findings with children from other cultures would increase our confidence in the transcultural generalizability of SCT’s internal-external validity.