Kraft lignin (KL) was oxidized by peracetic acid, which is generated by mixing acetic acid and hydrogen peroxide, to produce polycarboxylates for use as a plasticizer for cement paste. Peracetic acid cleaves the aromatic ring structure of KL and introduces carboxylate groups with ring-opened chain structure. After oxidation, the water-soluble fraction (Cx-lig) was obtained, and the performance of the Cx-lig as a plasticizer was compared with two commercial plasticizers, lignosulfonate (LS) and polycarboxylate ether (PCE). In mortar table tests, the increase in cement fluidity with the Cx-lig was greater than with LS and PCE. Fourier-transform infrared spectroscopy, carbon-13 nuclear magnetic resonance, gel permeation chromatography, elemental analysis, and charge density analysis were used to determine the structure of the Cx-lig. Considering all the results, the Cx-lig had a polycarboxylate structure containing numerous carboxylate groups, and their high charge density was the key factor that caused the Cx-lig to increase the cement fluidity more than LS or PCE.