Pseudarchaster alascensis has marginal plates in a continuous band around the perimeter of the aboral surface, and the joints between plates are ill-defined. It is reddish-orange aborally and paler on the oral side. Its five arms can grow up to 10 cm long. The ratio of arm to disc is from 2.6 to 4.0. The aboral tabulate plates are small and hexagonal, forming a relatively smooth surface. Each plate is topped with 15 to 22 granules and has 6 papulae around it. P. alascensis has 33 to 58 superomarginals that encroach well onto the aboral surface, but the individual plates are not well defined. Compact hexagonal granules cover each plate. The corresponding inferomarginals have a few broad, pointed spinelets, as well as granules. The oral intermediates form a triangular area comprised of rows of plates connecting between adambulacrals and inferomarginals up to the 10th or 17th inferomarginal. Each plate is armed with unequal, crowded granules, the central ones flat and lance-shaped, as on inferomarginals. The adambulacrals bear 5 or 6 flat furrow spines, and some shorter, stouter spines continue around the edge of each plate. The oral surface has 1 to 3 more robust, bluntly pointed spines. The adjoining mouth plates share a single large apical spine, and each has about 7 marginal spines.
Similar SpeciesPseudarchaster alascensis might be confused with Mediaster aequalis or Gephyreaster swifti. Compare the descriptions and illustrations of the marginal plates in these species.