Is an octopus a vertebrate

No. Because an octopus has no internal skeleton or spine/back bone, it is classified as an invertebrate.

More information:

The octopus is a mollusk, a grouping with many various characteristics. Squid are also mollusks. Both fit into the definition of invertebrates because they do not have an internal skeleton with a spine. For more information about mollusks, see the related links with this question.