Academic self-concept and achievement have been found to be reciprocally related across time. However, existing research has focused on self-concept and achievement scores that have been averaged over long time-periods. For the first time, the present study examined intraindividual (within-person) relations between momentary (state) self-concept and lesson-specific perceived achievement (i.e., self-reported comprehension) in students' everyday school life in real time using intensive longitudinal data. We conducted an experience-sampling (e-diary) study with 372 German secondary school students in Grades 9 and 10 over a period of 3 weeks after each mathematics lesson. Multilevel confirmatory factor analyses confirmed a two-factor between-level and within-level structure of the state measures. We used dynamic structural equation modeling to specify a multilevel first-order vector autoregressive model to examine the dynamic relations between self-concept and perceived achievement. We found significant reciprocal effects between academic self-concept and perceived achievement on a lesson-to-lesson basis. Further, we found that these relations were independent of students' gender, reasoning ability, or mathematics grades. We discuss implications for methodology, theory, and practice in self-concept research and educational psychology more generally. Educational Impact and Implications Statement This study suggests that students' momentary perception of their mathematics ability (i.e., their state mathematics self-concept) directly influences their lesson-specific comprehension (i.e., perceived achievement) from mathematics lesson to mathematics lesson. In turn, state mathematics self-concept is itself influenced by students' previous perceived achievement (i.e., in showing reciprocal relations). Therefore, our results indicate that students' state mathematics self-concept makes a substantial contribution to their academic development in their everyday life at school.