While numerous studies have investigated body image, including body weight perception, most of which have focused on adolescents or young women, few studies have attempted to evaluate body weight perceptions in adult women according to age groups. This study was conducted to investigate the accuracy of self-perceived weight and actual body mass index (BMI) values among adult Korean women according to age. We used data from the 2016 Korean Study of Women's Health Related Issues, a population-based, nationwide, cross-sectional survey. BMI was calculated from self-reported weight and height. Participants were asked to describe their body image by choosing one of the following descriptions: very underweight, underweight, about right, overweight, or obese. The proportions of women aged 20-29, 30-39, 40-49, 50-59, 60-69, and 70-79 years who underestimated their body weight relative to their actual BMI category were 12.6%, 15.1%, 22.2%, 34.0%, 45.6%, and 50.7%, respectively; those who overestimated their body weight comprised 18.7%, 17.8%, 14.3%, 10.8%, and 7.4%. In all BMI categories, the proportion of those who overestimated their weight status increased as age decreased, while those who underestimated their weight status increased as age increased. After adjusting for possible covariates, age was strongly associated with both underestimation and overestimation. The odds ratio for underestimating one's weight status among women aged 70-79 yeas was 2.96 (95% CI: 2.10-4.18), and that for overestimation was 0.52 (95% CI: 0.35-0.79), compared to women aged 20-29 years. Age is the most important factor associated with weight perceptions among Korean women, affecting both underestimation and overestimation of weight status.