There is evidence of a relation between the approximate number system (ANS) and later mathematics achievement. Researchers have proposed various mediators of this relation, including executive functioning (EF), numeral knowledge, and mathematical language. The goal of the present study was to determine which factors mediate the relation between preschoolers' ANS and the change in their mathematics achievement over a five-month period. We collected data from 125 preschoolers ("mean age" = 4.2 years) in the fall and spring. We tested mediation models using path analysis models, controlling for children's fall mathematics achievement, age, sex, and parent education. EF was not a statistically significant predictor of mathematics achievement when controls were included. Numeral knowledge was not a significant mediator, but mathematical language was a significant mediator in both individual mediation and multiple mediation models. Although children's ANS predicts their later mathematics achievement, the relation is fully mediated by children's mathematical language.