I’ve long wanted it to be octopi but my iPhone and most sources say no. Out of the dictionaries, only Oxford English seems to include but says based on a misunderstanding. Some detail on history here:

https://en.m.wikipedia.org/wiki/Octopus under pluralisation

Interestingly:

> Historically, the first plural to commonly appear in English language sources, in the early 19th century, is the latinate form "octopi"

That’s quite late and interesting the “wrong” one was first.