Abstract Background Mammalian spermatozoa become fully motile and fertile during transit through the luminal fluid of the epididymis. At least 200 proteins are present in the epididymal lumen, but the potential roles of these luminal proteins in male fertility are unknown. Investigation of the function of these proteins will elucidate the mechanism of sperm maturation, and also provide new drug targets for male contraception. We cloned RNase9 from a human epididymis cDNA library for characterization and analysis of its functions. Findings It was predicted that human RNase9 gene was located on chromosome 14q11.2 and encoded a 205 amino acids protein with a signal peptide of 26 amino acids at the N-terminus. The protein had eight conserved cysteine residues characteristic of the RNase A family members and several potential post-translational modification sites. At the transcriptional level, RNase9 was expressed in a wide variety of tissues, and the expression was higher in men than in boys. RNase9 was localized to the post-equatorial region of the sperms' head. Immunofluorescence staining showed that RNase9 protein was present mostly in the epithelium of the epididymal tubule. Recombinant RNase9 had no ribonuclease activity. In addition, RNase9 had no detectable effect on sperm motility and fertilization as demonstrated by blocking spermatozoa with anti-RNase9 polyclonal serum. Conclusion RNase9 is expressed in a wide variety of tissues. It is located on the post-equatorial region of the sperm head and the epithelium of epididymal tubule. Although RNase9 belongs to the RNase A family, it has no ribonuclease activity.