High adherence rates to antiretroviral medications are necessary for people living with HIV/AIDS. The current study focuses on relationship-level predictors of HIV medication adherence by testing whether adherence rates differ by dyadic serostatus (seroconcordant vs. serodiscordant couples) among individuals with HIV in romantic relationships. Results showed a significant interaction between dyadic serostatus and relationship duration on adherence, such that individuals in long-term serodiscordant relationships reported better adherence than short-term serodiscordant relationships or seroconcordant partners in long-term relationships. Future research is needed to understand what relationship dynamics explain differences in adherence rates based on dyadic serostatus.