The purpose of this study is to investigate the mediating effect of self-encouragement in the relationship between perfectionism and therapeutic alliance of play therapists. A total of 104 play therapists in training or clinical settings participated in the study. Collected data were analyzed by SPSS 21.0 and Hayes’s Process Macro ver.4.0. The results are as follows. First, maladaptive perfectionism, therapeutic alliance, cognitive self-encouragement, and emotional self-encouragement showed significant correlations with each other. Second, emotional self-encouragement was found to play a mediating role in the relationship between maladaptive perfectionism and therapeutic alliance. This means that play therapists with high maladaptive perfectionism are less likely to encourage themselves emotionally, which in turn negatively affects the therapeutic alliance. This study reveals the mediating role of emotional self-encouragement and is meaningful in that it repeatedly emphasizes the need for continuing training to improve emotional self-encouragement so that play therapists can form a more effective therapeutic alliance and contribute to the foundation for the development of a self-encouragement program for play therapists.