Though it breathes through gills, the octopus is not a fish but a cephalopod marine mollusk. Also, unlike a fish, an octopus is an invertebrate, which means it doesn't have a backbone. It also has three hearts.
The octopus is considered a mollusk because it has a mantle that holds the organs it uses for breathing and excretion and has a rasping tongue, or radula. The octopus also shares features of its nervous system with other mollusks. It has no bones, and the hardest part of its body is its beak.
The octopus is also a cephalopod because its large head is merged with its arms.