Gold nanoclusters (AuNCs) are a unique class of materials that exhibit visible luminescence. Amorphous calcium phosphate (ACP) is a widely used biomaterial for a variety of purposes, such as drug delivery, bone cementing, and implant coatings. In this study, a nanocomposite of AuNCs and ACP is prepared by biomimetic mineralization in a Dulbecco's modified Eagle's medium (DMEM). The strong interaction between AuNCs and Ca2+ ions effectively induces aggregation of AuNCs. The as-formed nanocomposite, AuNCs@ACP, emits significantly enhanced luminescence compared to AuNCs alone. The luminescence enhancement mechanism is investigated using synchrotron X-ray absorption fine structure spectroscopy. In addition, the presence of AuNCs stabilizes ACP and also enhances the biocompatibility of ACP in promoting cell proliferation, and the nanocomposites are promising as nanoprobes for cancer therapy and/or bone tissue engineering.