The acrosome reaction is a regulated Ca²⁺-dependent secretion event required for sperm-egg interaction. Previous studies indicate that the process requires Rab3-dependent tethering of membranes, SNARE complex assembly, and Ca²⁺-mediated activation of synaptotagmin. Sperm are transcriptionally and translationally inactive; hence, most studies of the exocytosis mechanism are limited to membrane-permeant reagents. The effect of proteins involved in exocytosis has been assessed only in permeabilized cells. Polyarginine peptides are a powerful tool for delivering macromolecules to cells. Most reports indicate that membrane translocation of arginine-containing proteins requires endocytosis; therefore, this strategy might not be useful in sperm. However, our results indicate that GST and Rab3A, when fused with an arginine-rich peptide, were able to translocate into sperm. Moreover, membrane-permeant Rab3A initiated exocytosis when prenylated and activated with GTP. We show here that a key event after the cytoplasmic Ca²⁺ increase caused by progesterone is the activation of Rab3A. When active Rab3A is introduced into sperm, Ca²⁺ in the extracellular medium and in the cytoplasm is dispensable. However, a Ca²⁺ efflux from inside the acrosome is still required to achieve exocytosis. In conclusion, arginine-containing proteins can penetrate the sperm plasma membrane and thus are valuable tools to study sperm physiology in intact cells.--Lopez, C. I., Belmonte, S. A., De Blas, G. A., Mayorga, L. S. Membrane-permeant Rab3A triggers acrosomal exocytosis in living human sperm.