ABSTRACT: In South Africa, up to 40% of pregnant women are living with human immunodeficiency virus (HIV), and 30-45% are obese. However, little is known about the dual burden of HIV and obesity in the postpartum period. In a cohort of HIV-uninfected and HIV-infected pregnant women initiating antiretroviral therapy in Cape Town, South Africa, we examined maternal anthropometry (weight and body mass index [BMI]) from 6 weeks through 12 months postpartum. Using multinomial logistic regression, we estimated associations between baseline sociodemographic, clinical, behavioural, and HIV factors and being overweight-obese I (BMI 25 to <35), or obese II-III (BMI >35), compared with being underweight or normal weight (BMI <25), at 12 months postpartum. Among 877 women, we estimated that 43% of HIV-infected women and 51% of HIV-uninfected women were obese I-III at enrollment into antenatal care, and 51% of women were obese I-III by 12 months postpartum. On average, both HIV-infected and HIV-uninfected women gained, rather than lost, weight between 6 weeks and 12 months postpartum, but HIV-uninfected women gained more weight (3.3 kg vs. 1.7 kg). Women who were obese I-III pre-pregnancy were more likely to gain weight postpartum. In multivariable analyses, HIV-infection status, being married/cohabitating, higher gravidity, and high blood pressure were independently associated with being obese II-III at 12 months postpartum. Obesity during pregnancy is a growing public health concern in low- and middle-income countries, including South Africa. Additional research to understand how obesity and HIV infection affect maternal and child health outcomes is urgently needed.