Echinostrephus aciculatus is a species of sea urchin belonging to the family Echinometridae first documented by Alexander Emmanuel Rodolphe Agassiz in 1863.

[1] These are regular urchins of canonical form, with the test mostly spherical, bearing a pentaradial symmetry from the mouth at the center of the oral face to the anus on the aboral face.

The test measures from 2 to 4 centimetres (0.79 to 1.57 in) in diameter, and the color of the spines is very variable: they are often brown-purple, but can be shades of purple, brown, black, grey or off-white.

Those on the oral face can also have a cream color with brown-purple rings, although they are rarely visible.

The test ranges from black to light brown, with the bottom of the spines showing often highly contrasting rings.