The purpose of this study is to develop and validate the 21st century skills instrument for Korean elementary school students. From the literature review, a framework of 21st century skills was developed, and the instrument was developed based on the framework. We collected data for 496 5th and 6th elementary students and conducted exploratory factor analysis(EFA) and confirmatory factor analysis(CFA) to validate the instrument. Firstly, an EFA was employed with 249 participants to find the dimension of the construct of the instrument. As a result, 27-items were determined for the instrument, and the six dimensions were identified: knowledge construction, problem solving, responsibility, initiative and self-direction, socialization ability and social receptivity. Secondly, to confirm its validity and reliability, a CFA was employed with the other data from 247 participants. The result indicated that 21st century skills instrument for elementary school students was reliable and valid. Furthermore, to validate the external criterion validity, correlation analysis was conducted. The results showed resonable correlations among 21st century skills, communication skills, and problem solving skills. Therefore, it was concluded that the 21st century skills instrument for elementary school students was a reliable and valid instrument. Since the instrument will provide a tangible data by measuring the level of future competencies, it enables educators to establish strategic plans for transforming elementary education and for developing national human resources.