Adequate sperm motility is requisite for human fertilization, whereas the underlying causes or mechanisms of impaired sperm motility, asthenozoospermia, still remain largely unknown. RNASET2 (Ribonuclease T2) may be one of the effectors modulating human sperm motility. We determined if there is a correlation between RNASET2 expression levels in human semen from asthenozoospermia and fertile individuals. Thus, RNASET2 expression levels in spermatozoa and seminal plasma of healthy and asthenozoospermia individuals were evaluated using Western blot, laser scanning confocal microscope analysis, ELISA and flow cytometry. The results revealed that RNASET2 expression was identified in both human spermatozoa and seminal plasma. In spermatozoa from fertile individuals, it was localized to the acrosome, neck and the middle piece of tail regions. However, in spermatozoa from asthenozoospermia individuals (n = 67), RNASET2 staining was especially more frequent and evident in the neck and middle piece than that in fertile individuals (n = 59, p < 0.01). Similarly, higher RNASET2 expression was also apparent in seminal plasma from asthenozoospermia than in fertile individuals (p < 0.01). Moreover, purified RNASET2 had an inhibitory effect on sperm motility, especially on progressive motility (n = 23, p < 0.05). In conclusion, higher expression of RNASET2 in the semen of asthenozoospermia individuals may contribute to sperm motility impairment.