A novel microporous 2D Ni-based phosphonate metal-organic framework (MOF; denoted as IEF-13) has been successfully synthesized by a simple a green hydrothermal method and fully characterized using a large panel of experimental and computational techniques. From structural resolution from single-crystal X-ray diffraction, IEF-13 crystallizes in the triclinic space group P-1 based on bi-octahedra nickel nodes and a photo/electroactive tritopic phosphonate ligand. Remarkably, this material exhibits coordinatively unsaturated ni ckel(II) sites, free-PO3H2 and-PO3H acidic groups, a CO2 accessible microporosity, and an exceptional thermal and chemical stability. Further, its in-deep optoelectronic characterization evidences a photoresponse suitable for photocatalysis. In this sense, the photocatalytic activity for challenging H2 generation and overall water splitting has been pioneering evaluated for a phosphonate-MOF in absence of any co-catalyst using UV-Vis irradiation and simulated sunlight. IEF-13 is able to produce up to 2200 mol of H2•g-1 using methanol as sacrificial agent, whereas remaining stable and allowing recycling. preserving its crystalline structure. Even more, 170 mol of H2•g-1 were produced using IEF-13 as photocatalyst in absence of any co-catalyst in the more challenging overall water splitting, being this reaction limited by the O2 reduction. Thus, this original work opens new avenues to further optimize the photocatalytic activity of this type of multifunctional materials.