Largest organisms

The largest organisms found on Earth can be determined according to various aspects of an organism's size, such as: mass, volume, area, length, height, or even genome size. Some organisms group together to form a superorganism (such as ants or bees), but such are not classed as single large organisms. The Great Barrier Reef is the world's largest structure composed of living entities, stretching 2,000 km, but contains many organisms of many species.

This article lists the largest species for various types of organism, and mostly considers extant species. The organism sizes listed are frequently considered "outsized" and are not in the normal size range for the respective group.

If considered singular entities, the largest organisms are clonal colonies which can spread over large areas. Pando, a clonal colony of the aspen tree is widely considered to be largest such organism by mass. Even if such colonies are excluded, trees retain their dominance of this listing, with the giant sequoia being the most massive tree.