Previous studies on internationalization have shown that firms’ expansion into global markets will enhance their innovation. In this study, we examine how a firm’s domestic-overseas market split may affect its product innovation in the context of emerging markets. From the resource constraint perspective, we propose that there exists a negative relationship between a firm’s domestic-market split and its product innovation because resource and attention competition between domestic and overseas markets distracts the firm from paying sufficient resources and managerial attention to either market, thus hurting its product innovation. With a unique World Bank survey of 846 private manufacturing firms in China, the results support our arguments. Our theoretical development and empirical results contribute to a better understanding on how internationalization may affect firm innovation, especially in the context of emerging markets.