Objective Studies on duration of untreated psychosis are common in patients with schizophrenia, but few studies have investigated the relationship between duration of untreated illness (DUI) and suicide, especially in patients with chronic schizophrenia. Therefore, we intended to investigate the relationship between DUI and suicide and clinical correlates in patients with chronic schizophrenia.Methods A total of 1,555 Chinese patients with chronic schizophrenia were enrolled in this study. DUI was measured in years, reflecting the prolonged untreated periods observed in this population. Clinical correlates were assessed, including symptoms, cognitive functioning, and body mass index. Suicidal ideation and attempts were also examined. Statistical analyses, including multivariate models, were employed to investigate the associations between DUI and clinical correlates while controlling for potential confounders.Results The study revealed a significant proportion (23.3%) of patients with chronic schizophrenia in China received their first treatment after a 4-year delay, with the longest untreated duration reaching 39 years. Patients with longer DUI exhibited more severe negative symptoms, lower immediate memory scores, a higher likelihood of being overweight, and surprisingly, a reduced likelihood of suicidal ideation and attempts. Each additional year of untreated illness was associated with a 3% decrease in the risk of suicidal ideation and attempts.Conclusion The findings underscore the prevalence of extended untreated periods in Chinese patients with chronic schizophrenia and highlight the impact of DUI on negative symptoms, cognitive function, and body weight. Intriguingly, a longer DUI was associated with a lower risk of suicidal ideation and attempts.