A polyoxometalate-based inorganic–organic hybrid [WO4{Ni(en)2(H2O)}3] [Ni(en)3]{P2W18O62}·[Ni(en)3]CO3·H2O (1) (en = ethanediamine), was synthesized under hydrothermal conditions, and characterized by IR, elemental analyses, XPRD, UV diffuse reflectance spectroscopy and single crystal X-ray diffraction analyses. Compound 1 consists of a heterometallic cluster [WO4{Ni(en)2(H2O)}3]4+, two [Ni(en)3]2+ groups, one [CO3]2− group, one water molecule and a Wells–Dawson anion [P2W18O62]6− ({P2W18}), which represents the first POM-based ionic crystal with the heterometallic macrocation cluster. Photocatalytic studies indicate that compound 1 is not only actively photocatalytic for degradation of methylene blue (MB) under visible light irradiation, but very stable and easily separated from the reaction system for reuse as well.