Ecdysis is the molting of the cuticula in arthropods and related groups (Ecdysozoa). Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new covering of larger dimensions is formed. In preparation for ecdysis the arthropod will become inactive for a period of time, and then by crawling movements pushes forward in the old integument, which splits down the back allowing the animal to emerge. Often times this initial crack within the integumentary shell will be caused by an increase in blood pressure within the body (in combination with movement), forcing an expansion across its exoskeleton, leading to an eventual crack that allows for certain organisms such as spiders to extricate themselves from. For most organisms, the resting period is used as a stage of preparation in the secretion of fluid from the molting glands of the cellular layer and the loosening of the underpart of the cuticula occurs. Following the shedding of the old cuticula, a new layer is secreted during a second period of inactivity. All cuticular structures are shed at ecdysis, including the terminal linings of the alimentary tract and of the tracheae if they are present.

The molting of reptiles is sometimes called ecdysis.

Related to the word ecdysis is ecdysiast, a euphemism for a striptease dancer.