Penguins are birds.
The evidence is simple: they have feathers, lay eggs, breathe air, and care for eggs and chicks. Those traits place them among birds.
The confusion usually comes from two things. Penguins swim so well that people compare them with fish. They also look round, live near the sea, and often appear beside seals in Antarctic footage, which can make them feel mammal-like.
Living in water does not make an animal a fish
Penguins chase fish, krill, and squid at sea, but they cannot breathe underwater. They inhale before a dive, then rely on oxygen storage and oxygen-saving physiology while underwater.
That is very different from fish. For the diving version, read can penguins breathe underwater.
Flightless birds are still birds
Penguin wings did not vanish. Their job changed. Aerial flight was traded for underwater propulsion, which is the core of why penguins cannot fly.
For the broad beginner view, start with what penguins are, then move into the species guide.
FAQ
Are penguins mammals?
No. Penguins do not nurse young and do not give live birth. They lay eggs and protect the body with feathers.
Are penguins fish?
No. Penguins swim, but they breathe air with lungs and cannot breathe underwater through gills.
Can a flightless animal still be a bird?
Yes. Not all birds fly. Penguins turned wings into underwater propulsion tools.
