There is no one "right" answer per se. According to Merriam-Webster, both octopuses and octopi are acceptable plurals.

The reason for the octopi plural is because some people mistakenly associate octopus with Latin words like syllabus and alumnus. Since these words take an -i plural in the original Latin, and since Latin has enjoyed such a high status in the minds of English speakers for such a long time, people have attempted to maintain or reintroduce the Latin plural form for such words.

However, the word octopus is actually of Greek origin. The plural of octopus in Greek is, in fact, octopodes.

Now, I am not advocating using the form octopodes as the plural for this word. English borrows many nouns from other languages (Japanese, Russian, Arabic, and so on) and only rarely ever uses the morphological system of that language to create plural forms. (Even many borrowed Latin -us words do not ever take the -i plural suffix, like circus.)

So, you have three choices:

Use octopuses and pluralize it like any other average English word. Use octopi and force an etymologically inappropriate Latin suffix onto an innocent Greek word. Use octopodes and probably have to explain each time that "technically" this is the correct plural in Greek (which might be a hit at parties?).

As I said, there is no one right answer. Good luck to you!