Background: Sleep problems are common in adults with ADHD and may be bidirectionally associated with ADHD severity and other psychiatric symptoms. We investigated the prevalence of positive screenings for various sleep disorders, and their association with psychiatric comorbidities in a large sample of adults with ADHD from a specialized outpatient clinic. Methods: We included data of 3,691 adult patients diagnosed with ADHD, who had filled out a screener for sleep disorders (Holland Sleep Disorders Questionnaire (HSDQ)) as part of routine diagnostic assessment. The HSDQ screens for the sleep disorders insomnia, parasomnia, hypersomnia, circadian rhythm sleep disorders (CRSD), restless legs syndrome (RLS)/periodic limb movement disorder (PLMD), and sleep-related breathing disorders (SBD). As delayed sleep phase syndrome (DSPS) is very frequent in ADHD, we additionally screened for DSPS. Psychiatric comorbidities were diagnosed through clinical assessment and the Mini International Neuropsychiatric Interview (M.I.N.I.) Plus, which assesses 26 psychiatric disorders following the classification of the DSM-5. All data were retrieved from the electronic patient files. Results: Mean age was 35.4 and 49.4% of the patients were female. About 60% of the adults with ADHD screened positive for any sleep disorder. Highest prevalences were found for symptoms of DSPS (36%), insomnia (30%), and RLS/ PLMD (29%). Sleep problems in adults with ADHD were associated with comorbid depression, anxiety, substance use disorder, personality disorder, and post-traumatic stress disorder. Conclusion: Adults with ADHD often report sleep problems, which are associated with specific psychiatric comorbidities. Systematic screening for sleep disorders in adult patients with ADHD can contribute to a better understanding of their complaints and may aid improved and integrated treatment for the sleep and psychiatric problems.