Group efficacy is generally known to have a positive relationship with group effectiveness across various dimensions. However, while efficacy positively affects performance through motivational effects up to an optimal level, excessive efficacy may actually worsen performance due to overconfidence that exceeds actual capability. This study tested the hypotheses, through a longitudinal study, that an inverted U-shaped relationship exists between group efficacy and group performance, and that group cohesion and task interdependence would strengthen this relationship. Hierarchical regression analysis results showed that the inverted U-shaped relationship between group efficacy and performance was statistically significantly supported. At that point, group cohesion further strengthened the inverted U-shaped relationship, whereas task interdependence exhibited a moderating effect in the opposite direction of what was predicted. Accordingly, the significance and limitations of this study were discussed, and directions for future research were suggested.