The relation between reading and mathematics achievement is an active area of research, and evidence in the extant literature suggests they are closely related. However, there is a lack of understanding about how their relation develops over time. Indeed, traditional longitudinal analyses have focused on investigating each domain independently, which misses the opportunity to examine important relations between the two simultaneously as they change over time. Thus, in this study we investigate the joint development of reading and mathematics achievement and the association between their longitudinal trajectories modeled as piecewise linear phenomena using a Bayesian inference approach. We used a random sample (n = 1000) from the Early Childhood Longitudinal Study, Kindergarten class, which followed a cohort of children from kindergarten through middle school, where 49% were male and 76% were in public schools. The findings showed that both educational domains were positively associated along their piecewise linear trajectories, however, the magnitude of association decreased over time. Moreover, a focus on distinct developmental phases highlighted the reciprocal relation between reading and mathematics.