Objective: Symptoms of sluggish cognitive tempo (SCT) have been considered a potential subset of symptoms of ADHD, predominantly inattentive presentation (ADHD-I), or as a separate but related symptom dimension. We sought to characterize the relationships between SCT and both internalizing symptoms and executive functioning in adults with ADHD. Method: One hundred two adults diagnosed with ADHD completed clinical interviews and clinical rating scales. Hierarchical regression analyses were conducted to ascertain the independent predictive power of SCT symptoms for executive dysfunction after considering ADHD inattentive and hyperactive-impulsive symptoms and internalizing symptoms. Results: SCT was correlated with ADHD inattentive symptoms and dimensional measures of depression and anxiety symptoms, but not with clinical diagnosis of depression or anxiety disorder. SCT was independently predictive of executive function deficits over and above the effects of internalizing and ADHD symptoms. Conclusion: SCT in adults is associated with internalizing symptoms, ADHD inattentive symptoms, and, independently, with executive function deficits, particularly organization and problem solving.