The function of relaxin in human reproductive processes remains poorly understood. Nevertheless, relaxin is produced by the prostate in men and by the corpora lutea of ovaries and endometrium in women. Previous studies with contrasting results have suggested a possible role of relaxin on sperm function and fertilization. Here we show the expression of the relaxin receptor RXFP1 on the acrosome of human spermatozoa. Furthermore, relaxin induced the acrosome reaction and, interestingly, the percentage of viable sperm with the acrosome reaction induced by relaxin was similar to that obtained with progesterone. These data suggest a physiological role on sperm for relaxin produced by the prostate and/or the female genital tract.