A novel tetrahedral μ-AsO4-bridging hexadecanuclear Ni-substituted silicotungstate (ST) Na21H10[(AsO4){Ni8(OH)6(H2O)2(CO3)2(A-α-SiW9O34)2}2]·60H2O (1) was made by the reactions of trivacant [A-α-SiW9O34]10- ({SiW9}) units with Ni2+ cations and Na3AsO4·12H2O and characterized by IR spectrometry, elemental analysis, thermogravimetric analysis (TGA), and powder X-ray diffraction (PXRD). 1 contains a novel polyoxoanion [(AsO4){Ni8(OH)6(H2O)2(CO3)2(A-α-SiW9O34)2}2]31- built by four trivacant Keggin [A-α-SiW9O34]10- fragments linked through an unprecedented [(AsO4){Ni8(OH)6(H2O)2(CO3)2}2]9+ cluster, where the tetrahedral AsO4 acts as an exclusively μ2-bridging unit to link multiple Ni centers; such a connection mode appears for the first time in polyoxometalate chemistry. Furthermore, the electrochemical and catalytic oxidation properties of compound 1 have been investigated.