Adults with autism spectrum disorder often experience co-occurring mental health problems such as attention-deficit/hyperactivity-disorder, as well as impairments in executive function. Sluggish cognitive tempo, a cluster of behaviors including slow processing, daydreaming, and mental fogginess, has been shown to be associated with attention-deficit/hyperactivity-disorder. This study was designed to assess sluggish cognitive tempo in young adults with autism spectrum disorder. Specifically, we sought to establish a preliminary estimate of clinically significant sluggish cognitive tempo symptoms and to better understand its phenomenology and associations with executive function and psychiatric symptoms in the context of autism spectrum disorder. Young adults with autism spectrum disorder (n = 57; age 16-25 years; 84.2% male) completed a laboratory-based executive function task, and parents and participants completed measures of the participants' sluggish cognitive tempo, attention-deficit/hyperactivity-disorder, depression, and anxiety symptoms. Nearly one-third of the sample exhibited clinically impairing levels of sluggish cognitive tempo. Although sluggish cognitive tempo and attention-deficit/hyperactivity-disorder symptoms were significantly correlated, findings suggest the constructs are distinct. Results also suggest that increased sluggish cognitive tempo is related to executive function impairment and depression, but not anxiety symptoms. Considerations for assessment and long-term impacts of sluggish cognitive tempo for adults with autism spectrum disorder are discussed.