Lymantria dispar, also known as the gypsy moth or the spongy moth, is a species of moth in the family Erebidae native to Europe and Asia. Lymantria dispar is subdivided into several subspecies, with subspecies such as L. d. dispar and L. d. japonica being clearly identifiable without ambiguity. Lymantria dispar has been introduced to several continents and is now additionally found as an invasive species in Africa, North America and South America. The polyphagous larvae live on a variety of deciduous and coniferous trees and can cause severe damage in years of mass reproduction. Due to these features, Lymantria dispar is listed among the world's 100 worst invasive alien species.

The etymology of "gypsy moth" is not conclusively known; however, the term is known to have been in use (as 'Gipsey') as early as 1832.

Moths of the subfamily Lymantriinae are commonly called tussock moths due to the tussock-like tufts of hair on the caterpillars.

The name Lymantria dispar is composed of two Latin-derived words. The generic name Lymantria means 'destroyer'. The species epithet dispar means 'to separate' in Latin; it refers to the sexual dimorphism observed in the male and female imagines.

In July 2021 the Entomological Society of America decided to remove the name "gypsy moth" from its Common Names of Insects and Related Organisms List as "hurtful to the Romani people", since gypsy is considered an ethnic slur by some Romani people. In January 2022, the new common name "spongy moth" was proposed, as a translation from the French name "spongieuse" for the species, referring to the sponge-like egg masses laid by L. dispar.

The European native, and introduced North American, L. dispar moths are considered to be the same subspecies, L. d. dispar. Confusion over the classification of species and subspecies exists. The U. S. Department of Agriculture defines the Asian subspecies as "any biotype of L. dispar possessing female flight capability", despite L. d. asiatica not being the only accepted subspecies that is capable of flight. Traditionally, L. dispar has been referred to as "gypsy moth" even when referring to Japanese, Indian and Asiatic populations.

The European subspecies (L. d. dispar) is native to temperate forests in western Europe. It had been introduced to the United States in 1869, and to Canada in 1912.

The Asian subspecies (L. d. asiatica) is native to temperate Asia east of the Ural mountains. Since the early 1990s it has also been detected along the West Coast of temperate North America. From Southern Europe it is spreading northwards into Germany and other countries, where it hybridizes with the European subspecies, L. d. dispar.^{[citation needed]} A colony had been reported from Great Britain in 1995.^{[citation needed]}