Garcinol, a benzenetriol compound extracted from Garcinia cambogia, has antitumor activity, however, its antitumor mechanism remains unclear. The aim of this study was to investigate the role and mechanism of garcinol as a novel potential proteasome inhibitor. We applied the drug affinity responsive target stability (DARTS) method coupled to mass spectrometry to determine the binding protein of garcinol; the proteasome activity assay was used to determine the effect of garcinol on its hydrolase activity; immunofluorescence and proximity ligation assay (PLA) were used to detect the effects of garcinol on ubiquitin and RPN6; and flow cytometry were used to determine the effects of garcinol on cell apoptosis; and the anti-cancer effect was studied in organoid models. The results showed that RPN6 was a direct binding protein of garcinol; garcinol inhibited the hydrolase activity of proteasome, and induced the accumulation and aggregation of ubiquitin protein, and its proteasomal inhibitory effect was dependent on RPN6; further studies showed that garcinol induced oligomerization of RPN6 and formation of granules in the nucleus; finally, it was verified that garcinol induced apoptosis of tumor cells, and inhibited the growth of organoids of Apc^min/+ small intestine mice. These results suggest that garcinol is a potential proteasome inhibitor, which inhibits proteasome activity by directly targeting RPN6 on proteasome 19S, which in turn induces cell apoptosis and inhibits tumor growth.