Previous research suggests that people living with HIV (PLWH) sometimes internalize HIV-related stigma existing in the community and experience feelings of inferiority and shame due to their HIV status, which can have negative consequences for treatment adherence. PLWH’s interpersonal concerns about how their HIV status may affect the security of their existing relationships may help explain how internalized stigma affects adherence behaviors. In a cross-sectional study conducted between March 2013 and January 2015 in Birmingham, AL, 180 PLWH recruited from an outpatient HIV clinic completed previously validated measures of internalized stigma, attachment styles, and concern about being seen while taking HIV medication. Participants also self-reported their HIV medication adherence. Higher levels of HIV-related internalized stigma, attachment-related anxiety (i.e., fear of abandonment by relationship partners), and concerns about being seen by others while taking HIV medication were all associated with worse medication adherence. The effect of HIV-related internalized stigma on medication adherence was mediated by attachment-related anxiety and by concerns about being seen by others while taking HIV medication. Given that medication adherence is vitally important for PLWH to achieve long-term positive health outcomes, understanding interpersonal factors affecting medication adherence is crucial. Interventions aimed at improving HIV treatment adherence should address interpersonal factors as well as intrapersonal factors.