Social assertiveness and prosociality are two related but distinct aspects of social competence that are important to youth's social success. They are likely co-occurrence and interact dynamically over time, but their interdependence and interaction have been overlooked in previous research. This study examined the joint developmental trajectories of the two aspects from middle childhood to early adolescence and tested their associations with aggression, shyness, academic performance and depressive symptoms. Participants included a sample of elementary school children in China (initially N = 1012, 51.2% girls, initial M = 9.49 years). Data were collected each year for 4 consecutive years from self-reports and peer nominations. The parallel-process latent class growth model identified three balanced and two imbalanced trajectories: (1) low-stable assertiveness and low-increasing prosociality, (2) both moderate-stable, (3) both high-stable, (4) low-stable assertiveness and high-increasing prosociality, and (5) high-increasing assertiveness and high-decreasing prosociality. In addition, academic performance was a promoting factor whereas the depressive symptom was a weakening factor for the co-developmental trajectories; shyness was positively associated with the two imbalanced developmental groups; aggression was positively associated with the assertiveness-predominant group and negatively associated with the prosociality-predominant group. These findings reveal the heterogeneity of the co-developmental patterns of assertiveness and prosociality and underline the necessity for tailored optimal interventions of social competence according to the diverse characteristics of children.