Objective: Drug addiction is one of the most important problems that is increasing all over the world. Drug users are at high risk for infectious diseases, especially HIV/AIDS. The aim of this study was to determine the level of knowledge about HIV/ AIDS of drug users. Materials and Methods: This study was performed in a total of 151 drug users, who were arrested by the Narcotic Police Department and voluntarily attended the study in Mersin. The data, including socio-demographic characteristics, information about drug usage, and the drug user's knowledge about HIV/AIDS were obtained using a questionnaire. Results: The results showed that 56.3% of the drug users were aged 14-24 years; 92.1% were male; 47.6% were high school education, 46.4% were free workers; and 43% had multiple sexual partners. Of the study participants, 64.9% first used drugs when they were aged between 10-18 years, and 92.5% had used hashish. In addition, 92.1% of them answered correctly that HIV/AIDS is a contagious disease. The rates of drug users giving the correct answer regarding the blood and sexual transmission routes of HIV/AIDS were higher than for other transmission routes. The majority of drug users had acquired their knowledge about HIV/AIDS from the press. Conclusion: The results suggest that drug users need more knowledge about HIV/AIDS.