Lost Monarch, Coast redwood in Jedediah Smith Redwoods State Park, California.

Lost Monarch is a coast redwood in Jedediah Smith Redwoods State Park standing 320 feet tall with a diameter of 26 feet. It ranks among the five largest trees of its kind by total wood volume.

Lost Monarch was identified on May 11, 1998, by botanist Stephen C. Sillett and naturalist Michael Taylor in the Grove of Titans. Its wood volume of 34,914 cubic feet made it the world's fifth largest coast redwood; the Del Norte Titan was listed as the largest single-stem tree because Lost Monarch's base includes multiple stems.

The Grove of Titans takes its name from the giant redwoods around Lost Monarch, several of which were named by discoverers Stephen C. Sillett and Michael Taylor: El Viejo del Norte, Screaming Titans, Eärendil and Elwing, and Stalagmight. The names mix Spanish, Tolkien references and descriptions of the trees themselves.

The exact location of the tree is kept secret by park officials to prevent excessive foot traffic. This approach helps protect the surrounding forest ecosystem from damage.

A 2003 report estimated that Lost Monarch carries about 1,000 pounds (450 kg) of leatherleaf fern (Polypodium scouleri) mat on its trunk and branches. This fern growth adds habitat within the tree itself.