Updated project metadata. Inhibitors of the secretion of cancer exosomes, which promote cancer progression and metastasis, can not only accelerate exosome biology research but also offer therapeutic benefits for cancer patients. We identified sulfisoxazole (SFX) which inhibits exosome secretion from breast cancer cells. SFX, an FDA-approved oral antibiotic, showed significant anti-tumor and anti-metastatic effects in mouse models of breast cancer xenografts, reduced the expression of proteins involved in exosome biogenesis and secretion, and triggered co-localization of multivesicular endosomes (MVEs) with lysosomes for degradation. These findings provide a foundation for exosome-targeted cancer therapies and the mechanistic studies on exosome biology.