Sperm storage within the female reproductive tract is widespread among animal species with internal fertilization, such as the fish family Poeciliidae, wherein the females possess a sperm storage micropocket (SSP) that can store sperm for up to a year. However, the underlying mechanism and morphogenesis of SSP remain unclear. Here, using histological analysis, we found that the SSP of the western mosquitofish (Gambusia affinis) was associated with protrusions arising from the previtellogenic oocyte (PO). Similar protrusions occur in the PO of Poecilia reticulata, P. latipinna, Xiphophorus helleri, and X. maculatus, suggesting that such PO-derived protrusions might be a universal feature in Poeciliidae. These protrusions exhibit significant differences in structural composition compared to the cytoplasm. Furthermore, the area of these protrusions positively correlates with the total area of the oocytes. After maturing, the protrusions split from the oocytes, leaving behind sac-like structures composed of germinal epithelium, which later develop into the SSP for storing sperm. The bottom of the SSP is composed of a single layer of germinal epithelium. We describe and depict the protrusion formation process in the fish family Poeciliidae during the previtellogenic stage, elucidating the SSP morphogenesis.