Research suggests that individual differences in math abilities correlate with approximate representations of quantity that are supported by a primitive Approximate Number System (ANS). However, relatively little research has addressed the direction of this association in early childhood. Here we examined the development of the ANS and math ability longitudinally in 3- to 5-year-old children. Children were observed at three time points roughly six months apart; they completed a nonsymbolic numerical comparison task that measured ANS precision and a standardized math assessment. A series of cross-lagged panel models was then estimated to explore the associations between ANS precision and math ability over time. Bidirectional associations between ANS precision and math ability emerged: Early ANS precision was related to children's later math skills, and early math ability also significantly predicted children's later ANS precision. Evidence for mutual enhancement over time between the ANS and symbolic math ability adds to our growing understanding of the ANS and how the ANS and math knowledge interact.