A novel three-dimensional (3D) CdII coordination polymer, namely, poly[[μ2-4,4′-bis(2-methylimidazol-1-yl)-[1,1′-biphenyl]](μ2-5-methylisophthalato)cadmium(II)], [Cd(C9H6O4)(C20H18N4)]n or [Cd(MIP)(4,4′-BMIBP)]n, (I), was synthesized by the hydrothermal method using 5-methylisophthalic acid (H2MIP), 4,4′-bis(2-methylimidazol-1-yl)-[1,1′-biphenyl] (4,4′-BMIBP) and Cd(NO3)2·6H2O, and characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy and thermogravimetric analysis. Compound (I) exhibits a novel fivefold interpenetrating 3D diamondoid framework. Additionally, it shows fluorescence emission in the solid state and promising photocatalytic activities for the degradation of methylene blue (MB) in water at room temperature.