Background: Substance use and sleep problems are common in patients with psychotic disorders, but their associations in these patients have not been evaluated. We aimed to investigate associations between substance use and sleep problems in a large nationwide cohort of patients with a psychotic disorder. Study Design: This study is part of the Finnish SUPER study, which belongs to the Stanley Global Neuropsychiatric Genomics Initiative. In this cross-sectional, multicenter study, participants (N = 8616) were recruited from primary and specialized healthcare. Patients with schizophrenia, schizoaffective disorder, bipolar disorder, and psychotic depression were included. Information on current alcohol (Alcohol Use Disorders Identification Test-Concise) and cigarette use as well as on lifetime illicit drug use, including cannabis, benzodiazepines, amphetamines, and opioids, was collected using questionnaires. The sleep outcomes in our logistic regression analysis were short (≤6 h) and long sleep (≥10 h) duration, difficulties initiating asleep, early morning awakenings, fatigue, and poor sleep quality (SQ). Results: Self-reported substance use was associated with a higher prevalence of sleep problems. After adjustments with age, gender, diagnostic group, and living status, hazardous alcohol use (eg, poor SQ odds ratio [OR] = 1.80, 95% CI: 1.49 to 2.16, P <. 001), current smoking (short sleep duration OR = 1.28, 95% CI: 1.08 to 1.52, P =. 005), and lifetime benzodiazepine misuse (difficulties initiating sleep OR = 2.00, 95% CI: 1.55 to 2.48, P <. 001) were associated with sleep problems. Conclusions: Substance use was associated with sleep problems. Our findings underline the potential benefits of screening substance use when treating sleep problems in patients with psychotic disorders.