ABSTRACT Despite the revitalization of labeling theory in explaining deviant behavior, the theoretical validity of the proposition regarding the informal labeling process has been largely neglected. To examine the proposed informal labeling process within the labeling theory, longitudinal path analyses were conducted using structural equation modeling, drawing on a nationally representative sample of adolescents in South Korea. The results revealed that engagement in delinquent behavior at Wave 1 had a significant impact on how others informally labeled the individual at Wave 2. Also, informal labeling by others at Wave 2 significantly influenced the development of deviant self-identity at Wave 3, and this deviant self-identity increased the likelihood of engaging in subsequent delinquent behavior at Wave 4. Namely, the hypothesized informal labeling process in labeling theory was supported: informal labeling based on previous delinquent behavior → development of deviant self-identity → engaging in subsequent delinquent behavior among adolescents. The findings from this study highlight the theoretical importance of the informal labeling process in explaining delinquent behavior among adolescents from a longitudinal perspective. The research implications are discussed.