Men diagnosed with sexually transmitted infections (STIs) are at greater risk for Human Immunodeficiency Virus (HIV) infection and STIs reinfection. This study aimed to test the effectiveness of a brief human immunodeficiency virus (B-HIV) prevention program on HIV and sexually transmitted infection (STI) knowledge, perceived benefits of condom use, risk reduction self-efficacy, risk reduction behaviors, and reinfection rate among Thai men with STIs. A quasi-experimental design was conducted. Participants were selected from men with STIsymptoms. They were randomly assigned to a B-HIV prevention program or usual care, 100 each. The program consisted of 3 modules. Key messages for HIV prevention were sent weekly through Line. Outcomes were HIV and STIknowledge, perceived benefits of condom use, risk reduction self-efficacy, risk reduction behaviors (condom use, the number of sexual partners, and condomless sex), and STIreinfection rate. Independent t-test and binary logistic regression were performed. The B-HIV prevention program significantly increased HIV and STI knowledge and resulted in perception of greater benefits from condoms and greater risk reduction self-efficacy. Program participants used condoms more frequently with many types of partners, especially with casual partners and sex workers. The intervention group practiced condomless sex less frequently than the control group. The program did not improve participants' condom use with lovers/steady partners and did not decrease the number of sexual partners and STIreinfection rate at 3-month follow-up. A B-HIV prevention program could reduce the risk of HIV infection among male clients with current STIs by enhancing their condom use with casual partners and sex workers. Strategies to improve condom use with lovers/steady partners among this high-risk population is needed.