Knowledge about one's own cognitive processes is the basis for the monitoring and regulation of learning processes for effective reading comprehension. Taking verbal cognitive ability into account, the present study aimed to examine the developmental trajectories of reading comprehension and declarative metacognitive knowledge and their interrelationship from grade 5 to grade 8. A multivariate latent growth curve analysis was conducted using longitudinal data collected from secondary school students in Germany at four time points (grades 5, 6, 7, and 8). The participants showed developmental gains in both reading comprehension and declarative metacognitive knowledge during this time period. While a widening of gap was observed for reading comprehension between high- and low-achieving students from grades 5 to 8, there was no evidence of an increase or a decrease in the gap in declarative metacognitive knowledge during the study period. The initial level of reading comprehension and the later developmental gain were significantly predicted by early verbal cognitive ability. Verbal cognitive ability had a significant effect on initial declarative metacognitive knowledge but not on later developmental gains. Developmental changes in reading comprehension and declarative metacognition were significant and appeared to be parallel in slope. While early declarative metacognitive knowledge was significantly related to later developmental gains in reading comprehension, early reading comprehension was not significantly related to later developmental gains in declarative metacognition. This study's findings are discussed with respect to the impact of strategies (declarative metacognitive knowledge) on reading comprehension and on the developmental interplay between both constructs.