BackgroundInsomnia is common among patients with psychiatric disorders and affects the clinical presentation and outcomes of the psychiatric disorder. The presentation of insomnia varies according to the type and severity of the psychiatric disorder.ObjectivesThis cross-sectional study aimed to assess the prevalence and severity of insomnia in adult psychiatric outpatients attending Psychiatry and Addiction Medicine Hospital, Faculty of Medicine, Cairo University. The study included 390 patients diagnosed with the following disorders: depressive disorders (n = 126 (32.2%)), anxiety disorders (n = 69 (17.6%)), psychotic spectrum disorders (n = 67 (17.1%)), bipolar and related disorders (n = 50 (12.8%)), trauma and stress-related disorders (n = 46 (11.8%)), obsessive–compulsive disorder (n = 30 (7.7%)), and somatic symptoms disorders (n = 2 (0.5%)). Patients were assessed using the Sleep Disorder Interview, Pittsburgh Sleep Quality Index (PSQI), Insomnia Severity Index (ISI), and Global Assessment of Functioning Scale (GAF).ResultsAmong the assessed patients, 314 (80.5%) had insomnia, with a higher prevalence in females (91.5%) compared to males (67.2%). The prevalence of insomnia was 96% in depressive disorders, 97.1% in anxiety disorders, 49.3% in psychotic disorders, and 32% in bipolar and related disorders; while all patients with trauma and stress-related disorders, obsessive–compulsive disorders, and somatic symptoms disorders had insomnia.The severity of insomnia as measured by ISI was positively correlated with the global functioning score (r = 0.341, p = < 0.001) and negatively correlated with the duration of the psychiatric illness (r = − 0.285, p = < 0.001).ConclusionsInsomnia is highly prevalent in outpatients with psychiatric disorders.