Flightless birds are birds that can not fly. There are about 40 species in existence today, the best known being the ratites (ostrich, emu, cassowary, rhea and kiwi) and the penguins. Some birds evolved flightlessness in response to the release from predation, for example on oceanic islands, although this is likely not the case for the ratites as evolutionary origins suggest a continental biogeographical providence.

Two key differences between flying and flightless birds are the smaller wing bones of flightless birds and the absent (or greatly reduced) keel on their breastbone. (The keel anchors muscles needed for wing movement.) Flightless birds also have more feathers than flying birds.

New Zealand has more species of flightless birds (including the kiwis, several species of penguins, and the takahe) than any other country. One reason is that until the arrival of humans roughly a thousand years ago, there were no large land predators in New Zealand; the main predators of flightless birds were larger birds.