The structurally new, carbon-free pentadecanuclear Fe-containing polyoxometalate, Na-21[NaFe15(OH)(12)(PO4)(4)(A-alpha-SiW9O34)(4)]center dot 85H(2)O (Na-21-Fe15P4(SiW9)(4)), was synthesized using a facile one-pot, solution-based synthetic approach and systematically characterized by various spectroscopic techniques. Single-crystal X-ray diffraction reveals that the title complex is composed of two [F(e)4(A-aSiW(9)O(34))] fragments and two [Fe-3.5(A-alpha-SiW9O34)] fragments stabilized by four PO4 linkers in a tetrameric style with idealized T-d point group symmetry. When coupling with (4,4'-ditert-butyl-2,2'-dipyridyl)-bis(coumarin)-iridium(III) hexafluorophosphate ([Ir(coumarin)(2)(dtbbpy)]-[PF6]) photosensitizer and triethanolamine (TEOA) sacrificial electron donor, polyoxoanion Fe15P4(SiW9)(4) effectively catalyzed hydrogen production with a minimally optimized TON of 986, which represents, to our knowledge, one of the highest values among known Fe-substituted POM-catalyzed hydrogen production systems. Both a mercury-poisoning test and FT-IR characterizations proved the structural stability of Fe15P4(SiW9)(4) catalyst under photocatalytic conditions. The photocatalytic mechanism of the present hydrogen-evolving system was investigated by time-solved luminescence and static emission quenching measurements.