Therapeutic nanoparticles are often functionalized so that they can be targeted to a specific biological site. This study showed that unmodified gold nanoparticles could inhibit the proliferation of ovarian cancer cells by preventing mitogen-activated protein kinase signalling and reversing epithelial to mesenchymal transition. Furthermore, the gold nanoparticles inhibited tumour growth and metastasis in two mouse models of ovarian cancer, suggesting that unmodified gold nanoparticles — which are known to bind cysteine- and lysine-rich biological proteins — could have potential as an anticancer agent.