Abstract: Social media serves as a crucial channel for adolescents’ social interaction and information sharing. Scientifically and effectively assessing adolescents’ social media-specific epistemological beliefs holds significant educational implications for understanding how they perceive and acquire information in social media environments. A questionnaire survey was conducted among 661 high school students, and the collected data were subjected to item analysis, exploratory factor analysis, confirmatory factor analysis, and reliability and validity tests. The final Social Media-specific Epistemic Beliefs Questionnaire contained 19 items, covering four dimensions, namely Certainty of Knowledge, Simplicity of Knowledge, Source of Knowledge, and Justification for Knowing. Confirmatory factor analysis indicated that the four-factor model fitted well. The total score of the questionnaire was positively correlated with the scores of curiosity, cognitive maturity and information awareness. The Cronbach’s α coefficient, split-half reliability and test-retest reliability of the total questionnaire and each dimension were all good, meeting the requirements of psychometrics. The results of measurement invariance test across genders showed that there was no gender difference in the questionnaire scores. The Social Media-specific Epistemic Beliefs Questionnaire has good reliability and validity, and can be used in related research on Chinese adolescents.