The lack of an efficient, robust, and inexpensive water oxidation catalyst (WOC) is arguably the biggest challenge for the technological development of artificial photosynthesis devices. Here we report the catalytic activity found in a cobalt hexacyanoferrate (CoHCF) Prussian blue-type coordination polymer. This material is competitive with state-of-the-art metal oxides and exhibits an unparalleled long-term stability at neutral pH and ambient conditions, maintaining constant catalytic rates for weeks. In addition to its remarkable catalytic activity, CoHCF adds the typical properties of molecule-based materials: transparency to visible light, porosity, flexibility, processability, and low density. Such features make CoHCF a promising WOC candidate for advancement in solar fuels production.