Existing empirical evidence has well documented the role of social support in both physical and psychological well-being among various populations. In the context of HIV prevention, the rapid increase of studies on social support merits a systematic review to synthesize the current global literature on association between social support and HIV-related risk behaviors. The current review reveals a complex picture of this relationship across diverse populations. Existing studies indicate that higher levels of social support are related to fewer HIV-related risk behaviors among female sex workers and people living with HIV/AIDS and heterosexual adults in general. However, influences of social support on HIV-related risk behaviors are inconsistent within drug users, men who have sex with men and adolescents. These variations in findings may be attributed to different measurement of social support in different studies, specific context of social support for diverse population, or various characteristics of the social networks the study population obtained support from. Future studies are needed to explore the mechanism of how social support affects HIV-related risk behaviors. HIV prevention intervention efforts need to focus on the positive effect of social support for various vulnerable and at-risk populations. Future efforts also need to incorporate necessary structure change and utilize technical innovation in order to maximize the protective role of social support in HIV risk prevention or reduction.