Distinct from the motile flagellated sperms in animals and early land plants, the non-motile sperm cells of flowering plants are carried in the pollen grain to the female pistil. After pollination, a pair of sperm cells are delivered into the embryo sac by pollen tube growth and rupture. Unlike other walled plant cells with equilibrium between internal turgor pressure and mechanical constraints of the cell walls, sperm cells wrapped inside the cytoplasm of pollen vegetative cell have only thin and discontinuous cell walls. The sperm cells are uniquely ellipsoid-shaped, though it is unclear how they maintain this shape within the pollen tubes and after release. This study reports that genetic disruption of three endomembrane-associated cation/H+ exchangers (CHXs) specifically causes sperm cells to become spheroidal in hydrated pollen in Arabidopsis. Moreover, the released mutant sperm cells are vulnerable and rupture before double fertilization, leading to failed seed set, which can be partially rescued by depletion of the sperm-expressed vacuolar water channel. These results reveal a critical role and molecular mechanism of cell-autonomous osmoregulation of the sperm cell for successful double fertilization.