Abstract: A new cinctan echinoderm, Graciacystis ambigua gen. et sp. nov. from Cambrian Series 3 rocks of Spain, is described based on more than 100 articulated specimens that range from 6 to 14.5 mm in thecal length. This material shows that Graciacystis ambigua, while plastic in thecal shape, is highly conservative in its thecal construction, with a fixed number of marginal plates and very limited addition of plates in the stele and ventral membrane through ontogeny. Ventral swellings on marginal elements are absent from the smallest specimens and become gradually more marked during growth. A cladistic analysis shows Graciacystis to be a basal cinctan, more derived than Sotocinctus and the Trochocystitidae and as sister group to a large clade formed by Sucocystidae + Gyrocystidae. The determinate growth pattern seen in Graciacystis seems to be the general pattern for all cinctans.