RNASEL is a 2-5A-dependent endoribonuclease that is a component of the interferon-induced 2-5A system, which plays a crucial role in the antiviral and apoptotic activities of interferons. In humans, many polymorphic sites within the RNASEL gene have been associated with an increased risk of developing prostate cancer. Here, we obtained coding sequences for the RNASEL gene from 11 primates and found evidence that positive selection has operated on the C-terminal endoribonuclease domain and the N-terminal ankyrin repeats domain of the protein, domains that directly interact with virus (i.e., ankyrin repeats are responsible for receiving environmental signals, and the endoribonuclease catalyses the destruction of the pathogenic viral RNA). To extend this finding, we studied variation within this gene in modern human populations by resequencing alleles from 144 individuals representing four separate populations. Interestingly, the frequency of the 541D allele shows a negative association with the incidence rate of prostate cancer in worldwide populations, and haplotypes containing the 541D polymorphisms demonstrate signatures of positive selection. RNASEL variants having the 541D haplotype likely have a greater ability to defend against infections by viruses, thus the loss of this activity may be associated with the development of prostate cancer. We provide evidence that positive selection has operated on the RNASEL gene, and its evolution is correlated with its function in pathogen defense and cancer association.