Developing valid and reliable instruments to assess common content knowledge (CCK) is a prerequisite for determining and improving the content knowledge of preservice teachers (PSTs) and teachers. We report on the development and psychometric analysis of an instrument for assessing PSTs' gymnastics CCK for secondary physical education teaching settings in South Korea. Once content and face validity were established, the Rasch model was used to assess construct validity. Data were collected from 196 participants in six physical education programs. From an original pool of 25 question items, 24 questions showed a good fit with high consistency. The test items were considered slightly difficult for the sample population, but the range of item difficulty was reported as good. The Rasch analysis shows that the instrument is a valid and reliable measure to assess PSTs' gymnastics CCK in a South Korea setting. The instrument can be used in teacher education and professional development to assess the knowledge levels of PSTs and teachers. The gymnastics CCK instrument is specifically tailored to the culture and educational context of teaching secondary physical education in South Korea, and as such, using this test in another country may not be valid.