Reading and mathematical competencies are important cognitive prerequisites for children’s educational achievement and later success in society. An ongoing debate pertains to potential transfer effects between both domains and whether reading and mathematics influence each other over time. Therefore, the present study on N = 5185 students from the German National Educational Panel Study examined cross-lagged effects between reading and mathematics from Grades 5 to 12. The results revealed, depending on the chosen causal estimand, negligible to small bidirectional effects. Adopting a between-person perspective, students with higher mathematics scores at one point exhibited somewhat higher reading scores at the subsequent measurement. In contrast, when adopting a within-person perspective, both skills predicted longitudinal increases of the other skill in the lower grades but reversed effects in higher grades. Taken together, these findings not only demonstrate that transfer effects between reading and mathematics in secondary education tend to be small but also suggest different patterns of effects depending on the modeling choice.