Gunnera is a genus of herbs, some of them gigantic, the only member of the family Gunneraceae. The 40-50 species vary enormously in leaf size. Gunnera manicata, native to the Serra do Mar mountains of southeastern Brazil, is perhaps the largest species, with leaves typically 1.5-2 m (5-6 ft) wide, but exceptionally (as at Narrow-water Park, County Cork, Northern Ireland in 1903) up to 3.4 m (11 ft) in width, borne on thick, succulent leaf stalks (petioles) up to 2.5 m (8 ft) long. It germinates best in very moist (not wet) conditions and temperatures of 75 to 85°F (24-29°C).

Only slightly smaller is G. masafuerae of the Juan Fernandez Islands off the Chilean coast. They can have leaves up to 2.9 m (9 ft 5 in) in width on stout leaf stalks 1.5 m (5 ft) long and 11 cm (4.5 in) thick according to Skottsberg. On nearby Mas a Fuera, G. peltata frequently has an upright trunk to 5.5 m (18 ft) in height by 25-30 cm (10-12 in) thick, bearing leaves up to 2 m (6 ft 4in) wide. G. magnifica of the Colombian Andes bears the largest leaf buds of any plant; up to 60 cm (2 ft) long and 40 cm (16 in) thick. The succulent leaf stalks are up to 2.7 m (8 ft 10 in) long. The massive inflorescence of small, reddish flowers is up to 2.3 m (7 ft 6 in) long and weighs about 13 kg. Other giant gunnera species are found throughout the Neotropics and Hawaii.

Several small species are found in New Zealand, notably G. albocarpa, with leaves only 1-2 cm long, and also in South America, with G. magellanica having leaves 5-9 cm wide on stalks 8-15 cm long.

This genus was named after the Norwegian botanist Johann Ernst Gunnerus.