The internal/external frame of reference model (I/E model) posits that individuals' achievement-related self-concepts are formed through social comparisons (e.g., self vs. peers) within academic domains and dimensional comparisons (e.g., math vs. verbal) between distinct domains. A large body of research has supported the theorized pattern of positive within-domain and negative cross-domain effects of achievement on self-concept. However, research on moderators of these effects has been scarce. In this article, we report results from 2 samples of 7th graders (N[subscript sample1] = 1,045 and N[subscript sample2] = 1,966) in which we investigated whether students' perceptions of teachers' domain-specific competence in judging students' achievement moderated the relation between teacher-assigned grades and students' self-concepts in the I/E model. Structural equation modeling was used to estimate the I/E model, which also included teachers' perceived diagnostic competence in each domain (i.e., math and verbal) and 4 latent interaction variables derived from that competence and from school grades in both domains. Both studies found support for the predictions of the I/E model. The cross-domain effect on math self-concept was consistently moderated in both studies: The lower students perceived their teachers' competence in diagnosing math achievement, the stronger the negative path from verbal grade to math self-concept. Furthermore, the results tentatively imply that the positive within-domain effect of verbal grade on verbal self-concept is stronger when students' perceptions of teachers' diagnostic competence in judging verbal achievement are higher. The results extend knowledge on the conditions of academic self-concept formation and provide valuable insights into possible ramifications of teachers' judgment accuracy.