Metacognitive processes are crucial components of self-regulatory ability. The study examined the developmental relationship between metacognitive monitoring and reading comprehension using longitudinal data from the National Educational Panel Study (NEPS) in Germany. We used a cross-lagged panel analysis – in the framework of Structural Equation Modeling–to examine the reciprocal relationship between metacognitive monitoring (assessed as a retrospective judgement of one’s performance) and reading comprehension at three time points: grades 5, 7, and 9. For the overall group of participants, we consistently found longitudinal reciprocal effects between metacognitive monitoring and reading comprehension from grade 5 to 7 and grade 7 to 9. This implies monitoring accuracy leads to higher reading comprehension over time and vice versa. Although the accuracy of students’ judgement was positively and reciprocally related to reading comprehension, a multi-group cross-lagged panel analyses showed that excessive underestimation appeared to have a more hindering effect than excessive overestimation on later reading comprehension. However, moderate underestimation might be less detrimental than excessive overestimation. In addition to shedding light on the scientific debate on the complex developmental interplay between reading comprehension and metacognitive monitoring, theoretical and practical implications of the results are provided.