Posttraumatic stress disorder (PTSD) is highly prevalent within prison settings, yet is often unidentified and undertreated. Complex PTSD (CPTSD) has been recently formally recognised in the International Classification of Diseases 11th revision (ICD-11) diagnostic framework but has never been explored in prison settings. We aimed to establish the prevalence of ICD-11 PTSD and CPTSD in a UK prison sample using a validated instrument (the International Trauma Questionnaire). We also explored the associations of these two diagnoses with their traumatic antecedents and psychiatric comorbidities. Randomly selected male, sentenced prisoners in a large medium-security prison in south London (N = 221) took part in a clinical interview which assessed PTSD, CPTSD, trauma histories, and comorbid disorders. Multinomial logistic regression was performed to examine differences between those with PTSD or CPTSD, and those without symptoms. A total of 7.7% (95% CI 4.5-12) of the male sentenced prisoners met diagnostic criteria for ICD-11 PTSD and 16.7% (95% CI 12.1-22.3) for CPTSD. A diagnosis of PTSD was associated with more recent traumatic exposure, comorbid generalised anxiety disorder, alcohol dependence, and Cluster B personality disorder. A diagnosis of CPTSD was associated with complex trauma exposure antecedents (developmental, interpersonal, repeated, or multiple forms), and comorbid with anxiety, depression, substance misuse, psychosis, and ADHD. This study confirms that CPTSD is a very common and comorbid condition in male prisoners. There is an urgent need to develop trauma-informed care in prisons.