This study aimed to introduce an ego-resiliency questionnaire for preadolescents (the ER-P) by restructuring the ER89 using the data of 1398 preadolescents from the Panel Study on Korean Children. The ER-P was proposed as a 10-item second-order instrument with two factors (Optimal Regulation and Openness to Life Experiences). The ER-P achieved configural, metric, and scalar levels of measurement invariance across gender. The ER-P scores exhibited correlations with personality traits of the five-factor model and general happiness, providing evidence of its convergent validity. This suggests that the ER-P has sound psychometric properties for evaluating ego-resiliency in Korean preadolescents. The validation of the ego-resiliency questionnaire specifically tailored for younger individuals marks a significant expansion of the ego-resiliency research domain, especially considering that existing instruments were initially designed for adults. The ER-P will be useful in education, counseling, and research settings that require information on preadolescents' self-rated inner strength.