As lifelong antiretroviral therapy (ART) for pregnant women is implemented, it is important to understand the attitudes and norms affecting women's postpartum ART adherence. This is a qualitative cross-sectional study of HIV-positive postpartum women (n = 112) enrolled in a 2-year observational prospective cohort in Rwanda. Informed by the Theory of Reasoned Action (TRA), we conducted in-depth interviews with women whose children were 0–6, 7–12, 13–18, or 21–24 months of age to describe factors contributing to adherence and changes over time. Positive ART attitudes reported by women included mothers’ health promotion, prevention of infant HIV infection, higher CD4 count, and improved physical appearance. Negative attitudes were few, but included side effects and the lifelong nature of treatment. Learning from people living with HIV (PLHIV) was identified as a norm facilitating adherence; ART adherence was inhibited by fear of disclosure or stigmatization in communities and clinics. Poor adherence behaviors were common immediately after HIV diagnosis, not necessarily during prevention of mother-to-child transmission (PMTCT). Women with older children, most of whom stopped breastfeeding by 13–18 months, reported more barriers and missed doses than women with younger children. The TRA was useful in identifying the collective influence of attitudes, norms, and intentions on behavior. Findings suggest that HIV-positive women are vulnerable to poor adherence following HIV diagnosis and around the time of breastfeeding cessation. Lifelong treatment adherence can be supported through PLHIV exemplifying long-term ART use, fewer and less stigmatizing clinic visits, and counseling messages highlighting the benefits of drugs on appearance and illness prevention and incorporating biological feedback.