Abstract Overcrowding remains one of the most persistent obstacles to the delivery of correctional rehabilitation in Nigeria, yet facility-level evidence on how it undermines specific rehabilitation functions is still limited. This study examined the impact of prison congestion on inmate rehabilitation at Onitsha Medium Security Custodial Centre, Anambra State, a facility designed for roughly 210 inmates but which now holds more than 600, an occupancy rate of about 290 per cent. A mixed-methods design combined a structured questionnaire administered to 196 custodial officers, inmates and community members with semi-structured interviews involving 18 key informants and observational visits to the facility. Quantitative data were analysed using descriptive statistics and chi-square tests at the 0.05 level of significance, while interview data were analysed thematically. Congestion was found to have compromised vocational training, educational provision, psychological support, and healthcare delivery, leaving these functions accessible to only a small proportion of inmates. All three hypotheses were supported: congestion was significantly related to rehabilitation programme quality (χ² = 81.34, p < .001), to inmate psychological wellbeing (χ² = 76.22, p < .001), and rehabilitation effectiveness was significantly related to recidivism risk (χ² = 78.90, p < .001). The paper argued that decongestion, led principally by reform of pre-trial detention practice, is the precondition on which any meaningful rehabilitation reform in Nigerian custodial centres depends, and sets out policy and practice recommendations accordingly.