A novel 3D metal–organic framework (MOF) {[Co 2.5 (dcpp) ( μ 3 -OH) ( μ -H 2 O)]·H 2 O} n (H 4 dcpp = 4,5-bis(4′-carboxylphenyl)-phthalic acid) with unprecedented ribbon-shaped secondary building units (SBUs), has been synthesized and characterized by single-crystal X-ray diffraction, elemental analysis, and IR. In addition, the thermal stability and magnetic behavior of the complex were also investigated.