The tallest living thing on earth is the giant red wood tree. The tree is the sole living species of the genus Sequoia reaching up to 115.5 metres in height excluding the roots and up to 7.9 metres diameter at breast height.
1 Additional Answer
The tallest living thing would the Hyperion, a redwood tree, in Redwood National Park. It stands 378.1 feet and took the title from a tree called Stratosphere Giant.