Objectives: To evaluate the demographic and psychosocial characteristics of incarcerated offenders in a Nigerian prison and investigate their relationship with psychiatric morbidity. Design: A cross-sectional study employing the 30-item version of the General Health Questionnaire (GHQ-30) and the depression sub-scale of the Hospital Anxiety and Depression scale (HADS-Depression). Setting: A medium security prison located in Ilesa, a semi-urban community in Southwestern Nigeria. Subjects: Three hundred and three prison inmates. Results: Out of the 352 inmates, 49 (13.9%) refused to participate in the study. There was a preponderance of male offenders (96.4%), and majority (59.3%) was awaiting trial. Thirteen inmates (4.3%) had been imprisoned before (i.e., before current imprisonment). Less than half (34%) reported enjoying good mental health in prison while 11 (3.7%) gave history of previous mental disorder before imprisonment. Majority reported the level of visitation and financial assistance by relatives and friends as poor. Also, majority reported prison accommodation and feeding as poor while less than a half of the sample were satisfied with the level of available health care. Majority of the inmates scored above the cutoff points on the measures of psychopathology. These were 87.8% and 85.3% on the GHQ-30 and the HADS-Depression Sub-scale respectively. Significant relationship was observed between GHQ-30 'caseness' and awaiting trial status, and also with reporting current mental health; prison accommodation and prison feeding as poor. In addition, significant depressive symptoms were associated with being a Muslim or traditional religion worshipper; lower education; single, divorced or separated marital status, and appraisal of current mental health as poor. Conclusion: The results are largely similar to findings from the developed world. Improving living condition and provision of basic needs of the prisoners appear essential for the promotion of their mental health.