Herein,　a　giant　Sb-rich　polyoxometalate　(POM)　{Sb21Tb7W56}　is　reported,　which　contains　the　largest　number　of　Sb　atoms　in　a　POM　so　far.　The　Sb-rich　POM　has　many　interesting　structural　features　and　is　a　rare　example　of　a　soluble　and　water-stable　giant　POM.　Biomedical　studies　indicate　that　the　Sb-rich　POM　exhibits　broad-spectrum　antitumor　activity　against　various　cancer　cell　lines　by　reactivating　the　P53-dependent　apoptotic　processes　and　disrupting　the　mitochondrial　membrane.　In　addition,　this　Sb-rich　POM　was　capable　of　suppressing　the　growth　and　metastasis　of　a　breast　cancer　in　vivo.　This　work　demonstrates　that　Sb-rich　POMs　are　promising　candidates　for　the　development　of　new　anticancer　drugs.