Liquid preservation of boar semen is a preferred method in pig husbandry, and antioxidants to protect against sperm oxidative stress during periods of storage have become the focus of recent research. Through its antioxidant activity, Isatis root polysaccharide (IRPS), a plant extract, can effectively reduce the cellular lipid peroxidation caused by the accumulation of reactive oxygen species inside mitochondria. In the present study, there was examination of the effects of no supplementation (Control) of a semen extender with or supplementation in different concentrations of IRPS (0.2, 0.4, 0.6, 0.8, and 1.2 mg/mL) on sperm quality variables and antioxidant capacity during liquid storage. The results indicate that after prolonged storage (≥ 3 days), the sperm motility was greater in the group supplemented with 0.6 mg/mL IRPS than in the other groups (P < 0.05). The use of this IRPS concentration also resulted in maintanence of acrosome integrity, plasma membrane integrity, mitochondrial membrane potential, and antioxidant capacity of the sperm (P < 0.05). Furthermore, the results of an in vitro fertilization study indicate IRPS at 0.6 mg/mL markedly increased the sperm fertilization capacity (P < 0.01) and embryonic development to the blastocyst stage (P < 0.05). The addition of 0.6 mg/mL IRPS enhanced the antioxidant capacity of boar sperm, resulting in greater preservation of sperm motility and fertilization capacity during liquid storage. These findings indicate that IRPS has the potential to be used as a component of a semen-preserving diluent to maintain sperm quality during storage. Copyright © 2019 Elsevier B.V. All rights reserved.