The betaretrovirus Mouse Mammary Tumor Virus (MMTV) is the well characterized etiological agent of mammary tumors in mice. In contrast, the etiology of sporadic human breast cancer (BC) is unknown, but accumulating data indicate a possible viral origin also for these malignancies. The presence of MMTVenv-like sequences (MMTVels) in the human salivary glands and saliva supports the latter as possible route of inter-human dissemination. In the absence of the demonstration of a mouse-man transmission of MMTV, we considered the possibility that a cross-species transmission could have occurred in ancient times. Therefore, we investigated MMTVels in the ancient dental calculus, which originates from saliva and is an excellent material for paleovirology. The calculus was collected from 36 ancient human skulls, excluding any possible mouse contamination. MMTV-like sequences were identified in the calculus of 6 individuals dated from the Copper Age to the 17th century. The MMTV-like sequences were compared with known human endogenous betaretroviruses and with animal exogenous betaretroviruses, confirming their exogenous origin and relation to MMTV. These data reveal that a human exogenous betaretrovirus similar to MMTV has existed at least since 4,500 years ago and indirectly support the hypothesis that it could play a role in human breast cancer.