Adolescents with autism spectrum disorder frequently experience social communication difficulties, executive functioning deficits, and anxiety and depressive symptoms, which are similar to the symptoms and correlates of sluggish cognitive tempo. Although sluggish cognitive tempo is related to, but distinct from, the inattentive and hyperactive-impulsive symptoms of attention-deficit/hyperactivity disorder that commonly co-occur with autism spectrum disorder, few studies have examined sluggish cognitive tempo in autism spectrum disorder. We examined whether sluggish cognitive tempo and attention-deficit/hyperactivity disorder were differentially associated with autism symptomatology, daily life executive functioning, and internalizing and externalizing symptoms in 51 adolescents (ages 13-18 years) with autism spectrum disorder without intellectual disability. Regression analyses controlling for age and IQ showed that sluggish cognitive tempo symptoms, but not attention-deficit/hyperactivity disorder symptoms, were associated with increased autism symptomatology and internalizing symptoms. Attention-deficit/hyperactivity disorder symptoms, but not sluggish cognitive tempo symptoms, were associated with increased externalizing behaviors and behavior regulation deficits. Both sluggish cognitive tempo and attention-deficit/hyperactivity disorder were independently associated with increased metacognitive deficits. This study provides preliminary evidence that sluggish cognitive tempo symptoms are elevated in autism spectrum disorder and associated with key clinical correlates, with implications for the assessment and treatment in adolescents with autism spectrum disorder.