Selenium (Se) is an essential trace element for human beings and many other forms of life. Organic selenium from natural foods has greater bioavailability and is safer than inorganic selenium species. In this article, the structural properties and antioxidant activities of a Glycyrrhiza uralensis polysaccharide (GUP) after selenylation modification were investigated. The GUP was extracted by water decoction and ethanol precipitation and purified via protein elimination using the trichloroacetic acid method and column chromatography. The purified product was subsequently modified by the nitric acid-sodium selenite (HNO3-Na2SeO3) method. The selenized GUP (SeGUP) product was characterized by Fourier transform-infrared (FT-IR) spectroscopy, and its thermal stability, particle size, and antioxidant activities were investigated. FT-IR analysis indicated that the selenium in SeGUP existed mainly as O-Se-O. The thermal stability and particle size of SeGUP differed significantly from those of GUP. Moreover, compared to GUP, SeGUP exhibited greater antioxidant activities in vitro and in vivo. These results indicate that selenylation modification significantly enhances the antioxidant activity of SeGUP, increasing its potential for application as an antioxidant. Graphical abstract ᅟ.