Starfish (also called sea stars) are invertebrates, not vertebrates. They belong to the phylum Echinodermata—the same group as sea urchins, sand dollars, and sea cucumbers—and they do not have a backbone or vertebral column.
NOAA notes that sea stars are not fish either: they lack gills, scales, and fins, and they circulate seawater through a water vascular system rather than blood. About 2,000 marine species exist worldwide. Their five-point radial symmetry is typical of echinoderms, though some species have many more arms.