Gold nanoparticles of different sizes, shapes, and decorations exert a variety of effects on biological systems. We report a novel mechanism of action of chemically modified, tryptone-stabilized gold nanoparticles (T-GNPs) in the triple-negative breast cancer (TNBC) cell line, MDA-MB-231. The T-GNPs, synthesized using HAuCl4.3H2O and tryptone and characterized by an assortment of spectroscopy techniques combined with high-resolution electron microscopy, demonstrated strong antiproliferative and anti-clonogenic potential against MDA-MB-231 cells, arresting them at the G1 phase of the cell cycle and promoting apoptosis. The molecular mechanism of action of these particles involved induction of unipolar clustering and hyper amplification of the supernumerary centrosomes (a distinctive feature of many tumour cells, including TNBC cells). The clustering was facilitated by microtubules with suppressed dynamicity. Mass spectrometry-assisted proteomic analysis revealed that the T-GNP-induced G1 arrest was facilitated, at least in part, by downregulation of ribosome biogenesis pathways. Due to the presence of supernumerary centrosomes in many types of tumour cells, we propose chemical induction of their unipolar clustering as a potential therapeutic strategy.