The measures presently available to examine the effectiveness of school counselors to provide counseling services to youth and emerging adults who identify as gender diverse are limited. This mixed-methods research study aimed to validate the psychometric properties of the Counselor Competence Gender Identity Scale (CCGIS) with a sample of school counselors. The CCGIS is a competency-based assessment that measures the effectiveness of counselors who provide services to gender diverse individuals in schools. We also examined school counselors’ levels of gender identity counselor competence in relation to region of practice. Considering requests by many gender diverse individuals to have gender-inclusive curriculum in the schools and the negative impact of the COVID-19 pandemic on gender minority youth, we recruited school counselors in the United States to take the CCGIS so its factor structure and validity could be examined. Exploratory factor analysis retained 25 items that formed four subscales with good internal consistency. Criterion and concurrent validity tests indicated that the CCGIS is psychometrically sound. The CCGIS was able to discriminate between groups of school counselors based on region. The CCGIS is a valid tool that can be self-administered by school counselors to assess their effectiveness with gender diverse students.