Prison system in Croatia is based on a rehabilitation concept, and purpose of prison sentence is to enable a person to live in freedom without committing criminal offenses and in accordance with social norms. This purpose is mostly achieved by psyhosocial treatment for prisoners. Drug users make up a large proportion of the population in Croatia’ prison system and they are particularly at-risk group because of their characteristics. In their treatment there are two main goals: to reduce criminal recidivism, but also drug use recidivism. Also, their addiction is often directly related with their criminal activity, they are more likely to do recidivism, to have medical issues and to do risky behavior in prison. Those are the reasons that effective psyhosocial treatment for this population is hard, but important work. This thesis is trying to present characteristics of drug addicts and their psychosocial treatment in the prison system of the Republic of Croatia. The introductory part presents the psychosocial treatment of addicts in different systems of the state, and then focuses on the prison system, the characteristics of drug addicts in that system and the psychosocial treatment that is carried out with them. Also, there are presented fhe most important documents on which this treatment is based, the most important changes in the psychosocial treatment of drug addicts in the last 10 years in the area of the prison system and the most effective psychosocial interventions for this population. At the end of the paper, suggestions are made for improving the quality and effectiveness of psychosocial treatment of drug addicts in the Republic of Croatia.