Female reproductive fluid, the fluid that surrounds the eggs, has attracted increasing attention for its role in fertilization and post-mating sexual selection through its effects on sperm traits. Surprisingly, however, only a few studies have investigated the effects of female reproductive fluid on the eggs. Yet, these effects might offer great potential to affect fertilization dynamics by, for example, increasing the opportunities for post-mating sexual selection. Here, we determined whether, by extending the egg fertilization window (time available for egg fertilization), the female reproductive fluid could also increase the opportunities for multiple paternity. Using the Zebrafish Danio rerio we first tested the prediction that female reproductive fluid increases the egg fertilization window, and then, using a split-brood design with sperm of two males added at different times after eggs activation, we tested whether the degree of multiple paternity varies in presence or absence of female reproductive fluid. Our results reveal the potential of the female reproductive fluid to increase multiple paternity throughout its effects on the egg fertilization window thus broadening our knowledge of the mechanisms females in externally fertilizing species affect post-mating sexual selection.