RATIONALE African Americans living with HIV are less likely to adhere to antiretroviral treatment (ART) compared to other racial/ethnic groups. Medical mistrust is thought to be a factor in this disparity. OBJECTIVE We examined (1) whether exposure to HIV conspiracy beliefs, a specific type of HIV-related mistrust (about the origins and treatment of HIV) in social networks is associated with ART nonadherence among African Americans living with HIV; and (2) the characteristics of individuals who discuss HIV-related mistrust in the social networks of African Americans living with HIV. METHODS At baseline and 6- and 12-months post-baseline, 175 African Americans living with HIV on ART completed egocentric social network assessments, from which we assessed the structure and composition of their personal networks (the social context immediately surrounding them). HIV-related mistrust was operationalized with an indicator of whether any social network member had expressed HIV conspiracy beliefs to the participant. Daily medication adherence was monitored electronically. RESULTS At baseline, 63% of participants agreed with at least one conspiracy belief, and 55% reported hearing at least one social network member ("alter") express conspiracy beliefs. In a multivariate linear repeated measures regression, expression of conspiracy beliefs by similar others in the network (in terms of age, gender, HIV status, sexual orientation, and race/ethnicity) was associated with ART nonadherence (i.e., percentage of prescribed doses taken). In a multivariate logistic regression, expression of conspiracy beliefs was more likely among social network members who were HIV-positive, who knew the participants' serostatus, and with whom participants interacted frequently, and less likely among more well-connected social network members. CONCLUSION HIV-related mistrust in the network may be most influential when expressed by similar others who may be HIV-positive themselves.