© 2015 Elsevier B.V. Background: Loneliness involves subjective, rather than objective, social isolation and has a range of negative effects on mental and physical functioning. The purpose of this study was to examine the prevalence of loneliness in psychotic disorders and its association with symptoms and cognitive performance. Method: Data were drawn from the second Australian National Survey of Psychosis and comprised responses from 1642 participants with an International Classification of Diseases 10 diagnosis of psychotic disorder who had completed a semi-structured interview of symptoms and social functioning (including loneliness), along with standardized assessments of current (digit symbol coding; DSC) and premorbid (National Adult Reading Test) cognitive ability. We examined the prevalence of loneliness across the diagnostic categories of psychosis, and its association with psychotic and non-psychotic symptoms and digit symbol coding scores. Results: The prevalence of loneliness was high, ranging from 74.75% in participants with delusional disorders to 93.8% in depressive psychosis, and was significantly higher than in the general population. Loneliness was also significantly associated with anhedonia and subjective thought disorder. Participants feeling socially isolated/lonely for company had significantly lower DSC scores than those who only felt lonely occasionally. Unexpectedly, participants who reported not feeling lonely had the lowest DSC scores. Conclusions: Loneliness is common across all psychotic disorders, particularly in depressive psychosis. It is specifically associated with ongoing loss of pleasure and disordered thoughts as well as impairment in current cognitive functioning. However, poor cognitive functioning is not inevitably associated with loneliness. Implications for personalized treatment of psychosis are discussed.