Sequoiadendron giganteum (giant sequoia; also known as giant redwood, Sierra redwood, Sierran redwood, Wellingtonia or simply big tree—a nickname also used by John Muir) is the sole living species in the genus Sequoiadendron, and one of three species of coniferous trees known as redwoods, classified in the family Cupressaceae in the subfamily Sequoioideae, together with Sequoia sempervirens (coast redwood) and Metasequoia glyptostroboides (dawn redwood). Giant sequoia specimens are the most massive trees on Earth. The common use of the name sequoia usually refers to Sequoiadendron giganteum, which occurs naturally only in groves on the western slopes of the Sierra Nevada mountain range of California.
The giant sequoia is listed as an endangered species by the IUCN, with fewer than 80,000 trees remaining. Since its last assessment as an endangered species in 2011, it was estimated that another 13–19% of the population (or 9,761–13,637 mature trees) was destroyed during the Castle Fire of 2020 and the KNP Complex & Windy Fire in 2021, events attributed to fire suppression, drought and global warming. Despite their large size and adaptations to fire, giant sequoias have become severely threatened by a combination of fuel load from fire suppression, which fuels extremely destructive fires that are also boosted by drought and climate change. These conditions have led to the death of many populations in large fires in recent decades. Prescribed burns to reduce available fuel load may be crucial for saving the species.