Background It remains a challenge to promote reproductive healthcare behaviors—including preconception care—among emerging adults. To effectively influence behavioral changes among emerging adults, it is necessary to understand their behavior according to their psychological steps. Objective This study aimed to describe the distribution of preconception care behavioral stages among emerging adults, using the framework of the precaution adoption process model (PAPM), and to examine factors associated with each stage. Methods This cross-sectional study used an online convenience sample of 929 participants aged 18–25 years in South Korea between March and October 2020. Behavioral stages of preconception care were assessed using the PAPM, and participants were classified into three groups (unaware, pre-adoption, and adoption of preconception care behaviors). Separate binary logistic regression models were used to compare each group with the adoption group as a reference. Results Compared with the adoption group, the unaware group was more likely to be male, to be sexually inexperienced, to use contraception inconsistently, to have no intention of future childbearing, and to report higher self-perceived knowledge of pregnancy-related issues. The pre-adoption group was more likely to be sexually inexperienced and to report lower perceived health status and lower levels of reproductive health-promoting behaviors. Conclusions This study described the behavioral stages of preconception care using the PAPM framework and identified characteristics associated with each stage. The findings may inform the development of future communication strategies; however, further research is required to evaluate their effectiveness.