The culinary-medicinal king oyster mushroom Pleurotus eryngii is known to contain ergothioneine, and its products, including fruiting bodies, mycelia, and solid-state fermented products (adlay and buckwheat), were prepared to study their antioxidant properties. Fruiting bodies, regular and Hi-Ergo mycelia, and fermented products contained 2.05, 1.68, 5.76, 0.79-0.80 mg/g of ergothioneine, respectively. On the basis of the results obtained, P. eryngii products had effective antioxidant activity, reducing power, and scavenging ability on 1,1-diphenyl-2-picrylhydrazyl radicals and chelating ability on ferrous ions. Hi-Ergo mycelia was the most effective in the first 3 antioxidant properties in addition to its ergothioneine content. In addition, fruiting bodies were more effective in all antioxidant properties than regular mycelia. For ethanolic and hot water extracts from mycelia and fruiting bodies, the correlation coefficients between total phenol contents and each antioxidant attribute were 0.483-0.921. Overall, P. eryngii products with high amounts of ergothioneine could be used beneficially as a functional food.