Large urchin, body up to 8 cm, the longer spines can reach 10 cm. It has 2 orders of spines, the longer are thick, smooth, triangular in section, brown reddish often with white bands. The body is covered with very short basal spines, white or brown.

Indo-Pacific.

Host of the commensal shrimp Gnathophyllum americanum.

Other species of sea urchins have thick spines (Phyllacanthus imperialis, Prionocidaris verticillata), but H. mammillatus is distinguished by the spines with triangular section. The similar species Heterocentrotus trigonarius has longer and darker spines.