Hoplophyrs oatesi was originally classified as part of the spider crab family (Majidae). It is now included in the family Epialtidae. The carapace is perfectly round and nearly spherical. The body and legs have numerous stiff, pointed spines. The body is translucent with light pink and white lines. This crab attaches soft coral polyps or branches, especially of Dendronephthya sp., to its carapace and legs as camouflage. It also camouflages itself on the surface of Dendronephthya sp. coral. The carapace width in the adult female is approximately 1 cm. This crab feeds on Dendronephthya sp. coral polyps, probably recycling the pigments to create the body coloring.
The distribution of this crab is in the tropical waters or coral reefs of the Indo-Pacific region. There are records of this species in the shallow marine reefs of southern Japan, Taiwan, the Philippines, and Indonesia. There have been several sightings of this species in the waters near the Hengchun Peninsula and on Lanyu (Orchid Island). The specimens of the National Museum of Natural Science were collected from Shuangshi Rock on Lanyu.