Both mathematics anxiety and metacognitive monitoring have been identified as associated with or predictive of individual differences in arithmetic achievement in primary school children. Although there are various theoretical reasons for an association between mathematics anxiety and metacognitive monitoring, it is unclear at the empirical level how these variables are interrelated and whether their interrelation impacts their respective associations with arithmetic achievement. Gaining insight into this interplay is of utmost importance for the design of targeted interventions. We used a panel longitudinal design to investigate mathematics anxiety, metacognitive monitoring, and arithmetic achievement skills in 127 typically developing 7- to 8-year-olds (second grade) and followed them up 1 year later (in third grade). As such, participants were in the middle of an important developmental period for arithmetic, as well as mathematics anxiety and metacognitive monitoring. Our preregistered analyses showed that mathematics anxiety and metacognitive monitoring were correlated concurrently and longitudinally. Early mathematics anxiety predicted later metacognitive monitoring, but not vice versa. However, this association was mediated by arithmetic achievement. Further, the association between early mathematics anxiety and later arithmetic achievement was mediated by metacognitive monitoring. The association between arithmetic achievement and metacognitive monitoring, on the other hand, was unique and specific, without mediation of mathematics anxiety. Arithmetic achievement was found to be a unique, powerful predictor over developmental time of both mathematics anxiety and metacognitive monitoring, over and above their respective autoregressive effects. These results emphasize the importance of arithmetic achievement in the development of mathematics anxiety, metacognitive monitoring, and their interrelations.