Overweight and obesity in adults are increasing globally and in South Africa (SA), contributing substantially to deaths and disability from non-communicable diseases. Compared to men, women suffer a disproportionate burden of obesity, which adversely affects their health and that of their offspring. This study assessed the changing patterns in prevalence and determinants of overweight and obesity among non-pregnant women in SA aged 15 to 49?years (women of childbearing age (WCBA)) between 1998 and 2017. This paper conducts secondary data analysis of seven consecutive nationally representative household surveys—the 1998 and 2016 SA Demographic and Health Surveys, 2008, 2010–2011, 2012, 2014–2015 and 2017 waves of the National Income Dynamics Survey, containing anthropometric and sociodemographic data. The changing patterns of the overweight and obesity prevalence were assessed across key variables. The inferential assessment was based on a standard t-test for the prevalence. Adjusted odds ratios from logistic regression analysis were used to examine the factors associated with overweight and obesity at each time point. Overweight and obesity prevalence among WCBA in SA increased from 51.3 to 60.0% and 24.7 to 35.2%, respectively, between 1998 and 2017. The urban-rural disparities in overweight and obesity decreased steadily between 1998 and 2017. The prevalence of overweight and obesity among WCBA varied by age, population group, location, current smoking status and socioeconomic status of women. For most women, the prevalence of overweight and/or obesity in 2017 was significantly higher than in 1998. Significant factors associated with being overweight and obese included increased age, self-identifying with the Black African population group, higher educational attainment, urban area residence, and wealthier socioeconomic quintiles. Smoking was inversely related to being overweight and obese. The increasing trend in overweight and obesity in WCBA in SA demands urgent public health attention. Increased public awareness is needed about obesity and its health consequences for this vulnerable population. Efforts are needed across different sectors to prevent excessive weight gain in WCBA, focusing on older women, self-identified Black African population group, women with higher educational attainment, women residing in urban areas, and wealthy women.