Objective: Sluggish Cognitive Tempo (SCT), characterized by lethargy and daydreaming, has most commonly been studied in community samples and in youth with Attention-Deficit/Hyperactivity Disorder (ADHD). Despite shared neurodevelopmental symptoms with ADHD, few studies have investigated SCT in Autism Spectrum Disorders (ASD). The current study investigated SCT symptoms in youth with ASD, ADHD, and comorbid ASD+ADHD to explore the relations between SCT and global and social impairment. Method: Caregivers of children and adolescents (n = 98; ages 6–17) diagnosed with ADHD (n = 46), ASD (n = 28), or ASD+ADHD (n = 24) completed measures of social impairment, SCT, and demographic variables. Results: All three clinical groups demonstrated comparable levels of SCT. Diagnosis and SCT independently contributed to parent-rated social impairment, while SCT and IQ, but not diagnosis, contributed to clinician-rated global functioning. Specifically, having comorbid ASD+ADHD, but not an ASD or ADHD diagnosis alone, significantly predicted greater social impairment. Conclusion: These results extend previous literature investigating SCT in ASD and provide evidence to suggest that SCT is associated with social and global impairment above and beyond the impairment associated with ADHD and/or ASD. These results may have implications for clinical assessment and treatment of ASD and ADHD.