<p>Although colony fusion occurs in termites in the field, its positive effects on fitness are not yet fully understood. Therefore, in this study, we conducted two types of colony fusion experiments to verify whether colony fusion promotes colony growth by mixing and rearing individuals from one to three incipient colonies of <i>Reticulitermes speratus</i> using two different mixing procedures. We found that artificial colony fusion had a significantly positive effect on the colony growth rate of offspring in both experiments. To our knowledge, this is the first study on termites to show that simple mixing of offspring from different colonies can enhance colony growth, suggesting that this colony growth-promoting effect is one of the factors that trigger the evolution of colony fusion. Our results also suggest that the evolution of colony fusion in termites plays the same role as the secondary evolution of polyandry and pleometrosis in eusocial Hymenoptera.</p>