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Plural of “octopus”

The plural of octopus is octopuses (some dictionaries also accept octopi).

Why

"Octopus" is Greek in origin, not Latin, built from "okto" (eight) and "pous" (foot) — so the popular Latin-style plural "octopi" is technically a mismatch: applying a Latin -us-to-i ending to a word that never was Latin in the first place. The linguistically "correct" classical plural, following actual Greek pluralization, would be "octopodes," though almost nobody uses it outside of etymology discussions.

In practice, "octopuses" — the fully anglicized, regular -s plural — is both the most common form in everyday and scientific writing and the one every major dictionary lists as standard, precisely because it sidesteps the Greek-vs-Latin mismatch entirely by just treating "octopus" as an ordinary English noun. "Octopi" remains widely used anyway, a case where a linguistically inaccurate plural became popular purely by analogy with genuinely Latin words like "cactus/cacti" and "fungus/fungi" that happen to look similar. (English didn't have a strong existing word for the animal before this Greek-via-Latin borrowing, unlike many everyday nouns that displaced an existing native term — octopuses were simply unfamiliar enough to English speakers before regular sea trade and marine science that no earlier word needed replacing.)

Usage note

For a marine biology publication or any context where precision matters, "octopuses" is the defensible default; "octopi" shows up often enough in relaxed conversation that most readers will accept it without a second thought, though a stickler for etymology will flag the Greek-versus-Latin mismatch, and "octopodes" belongs almost exclusively to deliberately playful or scholarly writing.

Dictionary editors have been explicit about this one for decades: Merriam-Webster and the Oxford English Dictionary both list "octopuses" as the primary, most standard plural, "octopi" as a common secondary variant despite its etymological inaccuracy, and "octopodes" as a rare, chiefly historical or humorous form — a rare case where major dictionaries openly rank the options by legitimacy rather than presenting them as equally standard. English didn't actually borrow "octopus" until the mid-1700s, arriving through scientific Latin natural-history writing rather than through everyday Greek-to-English word transfer, which is exactly why the word skipped straight to a Latin-flavored plural debate despite never having been a Latin word — a mismatch that likely wouldn't have happened had "octopus" entered English earlier, through spoken borrowing rather than written scientific classification.

Common real mistakes

The most common misconception is assuming "octopi" must be correct because it looks like other -us/-i plurals — the error is a reasonable pattern-matching mistake, but it doesn't hold up once you know "octopus" was never a Latin word to begin with. A lighthearted but genuine side note: informal writing sometimes jokes about an octopus missing an arm as a "heptapus" (seven-foot) by analogy with the word's own Greek construction, a playful coinage rather than a real dictionary word, but one that shows how transparently "octo-" and "-pus" are still understood as "eight" and "foot" by ordinary English speakers today.

The wider pattern

Compare this to "platypus," another Greek-derived word often mistakenly given a Latin-style "platypi" plural for the exact same reason — both words are frequently miscategorized as Latin because so many other common -us nouns genuinely are Latin, making "octopus" and "platypus" the two most commonly cited examples in linguistics discussions of this specific Greek/Latin plural-pattern confusion.

Examples

Related plurals

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