Helicoplacus (often misspelled Helioplacus) is the earliest well-studied fossil echinoderm. Fossil plates are known from several regions. Complete specimens were found in Lower Cambrian strata of the White Mountains of California.

The animal was a cigar-shaped creature up to 7 centimetres (2.8 in) long that stood upright on one end. Unlike more typical echinoderms such as sea stars, Helicoplacus does not have fivefold symmetry. Instead, there is a spiral food groove on the outside along which food was moved to a mouth that is thought to be on the side. The respiratory system appears to be primitive. Although the animal does not look like a typical echinoderm, the plates are composed of the characteristic calcareous plates known as stereom that are common to all echinoderms. The ambulacrum is similar to that of the Edrioasteroidea; as a result, Helicoplacoidea may belong to Pelmatozoa.