Eucalyptus regnans, known variously as mountain ash, swamp gum, or stringy gum, is a species of medium-sized to very tall forest tree that is native to the Australia states of Tasmania and Victoria. It is a straight-trunked tree with smooth grey bark, but with a stocking of rough brown bark at the base, glossy green, lance-shaped to curved adult leaves, flower buds in groups of between nine and fifteen, white flowers, and cup-shaped or conical fruit. It is the tallest of all flowering plants; the tallest measured living specimen, named Centurion, stands 100 metres tall in Tasmania.

Centurion is the name given to a single Eucalyptus regnans tree growing in Southern Tasmania, Australia, and the world's tallest known Eucalyptus. The tree was first measured by climber-deployed tapeline at 99.6 metres (327 ft) tall in 2008, and was subsequently re-measured to be 100.5 metres (330 ft) tall by ground laser in 2018. This discovery places E. regnans as the fourth-tallest tree species in the world after the coast redwood, the Himalayan cypress, and the Shorea faguetiana, and taller than both the Sitka spruce and Coastal Douglas Fir. It was discovered in August 2008 by employees of Forestry Tasmania while analysing the data collected by LiDAR system used in mapping and assessment of state forest resources.