This study aims to develop and validate the Principal Competency Inventory (PCI), an instrument used to assess a principal's leadership competencies that promote student learning in South Korea. An extensive review of prior work was conducted to understand the theoretical foundation related to school leadership and the competency construct of the PCI derived from the Behavioural Event Interview of principals. Then, the psychometric procedures used in developing and validating the PCI were followed. The significance of this study and its practical implications for the use of this new instrument was discussed. From a theoretical and practical standpoint, this study contributes to the school leadership field by describing development and validation procedures, identifying where evidence of the reliable assessment instrument is lacking, and utilising the advantage of the competency-based framework. Also, the analysis of this study provides international perspectives about school leadership practices, thus supplementing the prior Western-focused literature in this field. In addition, this study offers useful information for policymakers and principal leadership development program designers who want to assess and evaluate principal leadership competency.