We identified breast cancer-associated protein (BCA3) as a novel binding partner of Mason-Pfizer monkey virus (MPMV) protease (PR). The interaction was confirmed by co-immunoprecipitation and immunocolocalization of MPMV PR and BCA3. Full-length but not C-terminally truncated BCA3 was incorporated into MPMV virions. We ruled out the potential role of the G-patch domain, a glycine-rich domain located at the C terminus of MPMV PR, in BCA3 interaction and virion incorporation. Expression of BCA3 did not affect MPMV particle release and proteolytic processing; however, it slightly increased MPMV infectivity.