Marthasterias glacialis is a fairly large starfish with a small central disc and five slender, tapering arms. Each arm has three longitudinal rows of conical, whitish spines, usually with purple tips, each surrounded by a wreath of pedicellariae.

Marthasterias glacialis is a large starfish up to 70 cm across, commonly 25-30 cm. It has 5 narrow tapering arms with 3 rows of longitudinal spines along each arm. The spines are white, usually with purple tips. spines are surrounded by specialised minute modified spines (pedicellariae), which are used for protection and to gather food.

Marthasterias glacialis is a starfish of 30 to 40 cm in diameter but it may be up to 80 cm. Its surface is covered by linear papillae from which arise large conical spines surrounded by numerous pedicellariae easily visible under magnifying glass. The colour is variable: whitish, bluish, pink, purple, greenish-grey or brown.

Marthasterias glacialis is a large species of sea star with five arms. They grow up to 70 cm across, although more typically reaching a size of around 35 cm across. Their colour can vary from yellow to orange to brown to grey, although the tips of each arm are usually purple.

Marthasterias is a genus of starfish in the family Asteriidae. It is monotypic and the only species in the genus is Marthasterias glacialis, commonly known as the spiny starfish. It is native to the eastern Atlantic Ocean.