There is a need for instruments that can be used in correctional settings to measure changes in aggressive behaviors over a limited time period. This study aimed to validate an instrument (the Prison Adjusted Measure of Aggression, PAMA) that assesses specifically the past month’s aggressive behaviors and is adapted for use in correctional facilities. The psychometric properties of the self-rated and interview versions of the PAMA were explored and compared to those of two well-established measures of aggression: The Staff Observation Aggression Scale (SOAS); and the self-rate Aggression Questionnaire-Revised Swedish Version (AQ-RSV). The study group comprised 93 male and 59 female inmates, who were followed for two months. During the study, the prevalence of aggressive acts was observed and reported by SOAS. On two occasions, at monthly intervals, subjects reported their own aggressive behaviors using AQ-RSV and the self-report version of the PAMA; also, a psychologist conducted interviews according to PAMA. This study’s main finding was that the self-rated version of PAMA is a valid measure of different types and dimensions of aggression (physical and verbal aggression, hostility) and has acceptable psychometric properties. Therefore, PAMA could potentially be of value for use in correctional services evaluating aggression managing treatment interventions.