We investigated the factorial structure of preservice teachers' academic self-concept with regard to three domains of professional knowledge (content knowledge [CK], pedagogical content knowledge [PCK], and pedagogical/psychological knowledge [PPK]). We also analyzed the relation between preservice teachers' academic self-concept and their professional knowledge. The sample consisted of N = 631 German preservice secondary teachers in biology and physics. To analyze the factorial structure of the academic self-concept, we applied confirmatory factor analyses (CFAs) and compared different models of complexity. Results confirmed that preservice teachers' academic self-concept is empirically separable into CK, PCK, and PPK. Furthermore, the self-concept scales were positively related to the corresponding test scores in the professional knowledge domains. Our results revealed that preservice teachers' academic self-concept is differentiated at a very early state of teacher education and reflects CK, PCK, and PPK performance. Hence, self-concept measures seem to provide an alternative means to assess preservice teachers' professional knowledge in comparison with traditional performance measures.