Eucalyptus erythrocorys, commonly known as Illyarrie, Red-capped gum or Helmet nut gum, is a mallee from Western Australia. The common name Illyarrie is the Noongar peoples name for the plant.
It is a small tree typically growing to a height of 3 to 8 metres (10 to 26 ft). The bark is smooth with a creamy colour but can have a few rough brown coloured patches where it persists on the trunk instead of being shed. The adult leaves are dark green sickle-shaped.
It blooms between February and April and produces yellow flowers. The Illyarrie is notable for its big flowers which can be 5 cm across or more. They are bright yellow, being covered by a bright-red cap (operculum) in bud - giving the epithet erythrocorys (red-helmet). The stamens are arranged in 4 bundles and the fruits are ribbed with a red top.