The gentoo penguin is easily recognized by the wide white stripe extending like a bonnet across the top of its head and its bright orange-red bill. They have pale whitish-pink webbed feet and a fairly long tail – the most prominent tail of all penguins.

Gentoos reach a height of 51 to 90 cm (20 to 35 in), making them the third-largest species of penguin after the two giant species, the emperor penguin and the king penguin. Males have a maximum weight of about 8.5 kg (19 lb) just before molting, and a minimum weight of about 4.9 kg (11 lb) just before mating. For females, the maximum weight is 8.2 kg (18 lb) just before molting, but their weight drops to as little as 4.5 kg (9.9 lb) when guarding the chicks in the nest. Southern gentoo penguins reach 75–80 cm (30–31 in) in length. They are the fastest underwater swimming penguins, reaching speeds of 36 km/h (22 mph).