PURPOSE: A major global public health challenge is the continuance of new pediatric HIV infections primarily because of mother to child transmission of HIV occurring mainly in sub-Saharan African countries. The purpose of this study was to examine antiretroviral therapy (ART) refill adherence and its determinants among pregnant women living with HIV in Nigeria. METHODS: A retrospective review of pharmacy refill records was undertaken to examine adherence data on 275 pregnant women undergoing ART in 4 high-volume HIV treatment sites in Nigeria. A pharmacy refill adherence measure was used to assess medication refill behavior of pregnant women living with HIV who had received an ART refill during a period of 3 months. Medication-based ART refill adherence was categorized as % adherence (100% adherence) or % nonadherence (<100% adherence) to the ART refill scheduled dates. Refill appointments were scheduled on a 28-day cycle. Multivariable logistic regression analysis was performed. FINDINGS: Of the 275 women, 59.3% (95% CI, 53.1%-65.5%) were adherent to their ART refill schedule. Women who initiated ART during the third trimester of their current pregnancy had the lowest adherence rate of 30.8% (95% CI, 7.7%-53.8%) compared with women who commenced ART before conception or during the first or second trimester. The availability of a treatment support person was significantly associated with ART refill adherence. The odds of medication-based refill adherence were 2.9 times higher for participants who had a treatment support person (odds ratio = 2.9; 95% CI, 1.6-5.2; p = 0.001). IMPLICATIONS: Results indicate that having a treatment support person could contribute to improving ART adherence in pregnant women living in Nigeria.