Equisetum giganteum, with the common name Southern giant horsetail, is a species of horsetail native to South America and Central America, from central Chile east to Brazil and north to Southern Mexico.
It is one of the largest horsetails, growing 2–5 metres (6.6–16.4 ft) tall, exceeded only by the closely allied Equisetum myriochaetum (up to 8 metres (26 ft) relying on surrounding plants' support). The stems are the stoutest of any horsetail, 1–2 cm diameter (up to 3.5 cm diameter in some populations), and bear numerous whorls of very slender branches; these branches are not further branched, but some terminate in spore cones. Unlike some other horsetails, it does not have separate photosynthetic sterile and non-photosynthetic spore-bearing stems.
Populations from northern Chile with very stout stems up to 3.5 cm diameter have sometimes been treated as a separate species Equisetum xylochaetum, but this is not widely regarded as distinct.