The electrooxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a monomer for degradable bioplastic, is a promising strategy for biomass upgrade and yet requires well-designed catalysts with high efficiency and selectivity. Taking advantage of the open metal sites of metal-organic frameworks (MOFs), quasi-MOFs represent viable catalysts, but the poor designability and unpredictable structures hinder their development. In this work, a Ni-based quasi-MOF was rationally designed and synthesized by controlled ligand engineering. Compared to the fully occupied metal clusters in the pristine MOFs, the accessible Ni sites in quasi-MOFs can efficiently convert HMF to FDCA with remarkable Faradaic efficiency (99.2%) and FDCA selectivity (98.3%). In situ characterizations and mechanistic analysis revealed that the open Ni sites created by partial ligand disconnection in quasi-MOFs are critical to the formation of high-valent active species and HMF oxidation. Moreover, serving as the anode in an integrated electrolysis system, such a quasi-MOF can not only reduce the cell voltage for hydrogen generation but also produce high-purity FDCA with good yield, offering a new opportunity for the simultaneous production of high value-added chemicals and sustainable hydrogen.