This study aims to identify early childhood sociability trajectories and examine their longitudinal associations with behavior problems. Using a population-based data set presented by the Panel Study on Korean Children, sociability trajectories from age 2 to age 4 were identified by latent class growth analysis. Associations between sociability trajectories and behavior problems at first grade were examined with binary logistical regressions. Research Findings: Korean children developed one of the three sociability patterns: low (10.059%), moderate (66.185%), or high (23.756%). Children's behavior problems at first grade differed according to their early sociability trajectories. First graders with the low sociability trajectory in early childhood were vulnerable to internalizing problems, such as affective or anxiety problems. Practice and Policy: A persistent low preference for being with others observed in early childhood can be a meaningful marker in predicting later internalizing behavior problems.