Background: The aim of this study was to investigate the effectiveness of the harm reduction group therapy based on Bandura's self-efficacy theory on risky behaviors of sex workers in Kerman, Iran.Methods: A quasi-experimental two-group design (a random selection with pre-test and post-test) was used. A risky behaviors questionnaire was used to collect. The sample was selected among sex workers referring to drop-in centers in Kerman. Subjects were allocated to two groups and were randomly classified into two experimental and control groups. The sample group consisted of 56 subjects. The experimental design was carried out during 12 sessions, and the post-test was performed one month and two weeks after the completion of the sessions. The results were analyzed statistically.Findings: By reducing harm based on Bandura's self-efficacy theory, the risky behaviors of the experimental group, including injection behavior, sexual behavior, violence, and damage to the skin, were significantly reduced in the pre-test compared to the post-test (P < 0.010).Conclusion: The harm reduction group therapy based on Bandura's self-efficacy theory can reduce the risky behaviors of sex workers.