Adaptive functioning (AF) has been overlooked in the assessment of intellectual disability within the UK Prison Service, primarily because there are no reliable and valid measures of AF suitable for use with prison populations. This position is incompatible with the obligation of Her Majesty's Prison and Probation Service to provide specialist interventions for individuals with intellectual disabilities. This study assessed the psychometric properties of the Adaptive Functioning Assessment Tool (AFAT), a new measure of AF developed for use in prisons. The AFAT was completed on 61 participants (mean age 41, standard deviation 11.8). Internal consistency was tested using Cronbach's a. Construct validity was assessed using linear regression and a Receiver Operator Characteristic analysis. The findings provide evidence to support the AFAT being a reliable and valid measure of AF in prison populations.