BACKGROUND: Sexually transmitted infections (STIs) have always been considered as a health problem in the world. Also, vulnerable women (addicts, jail breakers, and prostitutes) are at the highest risk. According to the World Health Organization's (WHO) approach, the only effective way to prevent and control this disease is public health education, and high-risk and vulnerable groups should be prioritized in educational programs. The aim of the study was to investigate the effect of health belief model (HBM)-based education on changing the behaviors related to STIs in vulnerable women. MATERIALS AND METHODS: The present study is an intervention (field trial) study in vulnerable women. Convenience sampling method was used in this study and the sample size was determined to be 84 subjects. Through tossing a coin, the social support center was selected as the intervention group and the drop-in center as the control group. Quantitative data analysis was performed, and descriptive and inferential statistical methods were used. RESULTS: The mean scores of perceived threat, perceived benefits, perceived barriers, and perceived self-efficacy and changes in performance between the three measurement stages, as well as changes in the scores of perceived threat, perceived benefits, perceived barriers, and perceived self-efficacy over three measurement times were significantly different between the two groups (interaction) ( < 0.001). The mean score of performance 3 months after the intervention was significantly higher than before the intervention ( = 0.001). CONCLUSIONS: The present study confirmed the effectiveness of the HBM in promoting behavioral modifiers that lead to STIs. Therefore, educational interventions with emphasis on understanding the threats, benefits, barriers, and self-efficacy and, ultimately, performance improvement in relation to STIs are recommended.