Ecdysozoa (ɛkdɪsɵˈzoʊ.ə) is a group of protostome animals (Telford et al. 2008), including Arthropoda (insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes. A large study in 2008 by Dunn et al. strongly supported the Ecdysozoa as a clade, that is, a group consisting of a common ancestor and all its descendants.

The group is also supported by morphological characters, and can be considered as including all animals that shed their exoskeleton (see ecdysis). Groups corresponding roughly to the Ecdysozoa had been proposed previously by Perrier in 1897 and Seurat in 1920 based on morphology alone. The group has been contested by a significant minority of biologists. Some have argued for groupings based on more traditional taxonomic techniques (Nielsen 1995), while others have contested the interpretation of the molecular data (Blair et al. 2002, Wägele et al. 1999).