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 can be dirty brown to pale greyish green in colour with purple tips to the arms.Small individuals of Marthasterias glacialis may be confused with Leptasterias mulleri (Northern starfish).