With an average length of and an average height of , the Java mouse-deer is the smallest extant (living) ungulate or hoofed mammal, as well as the smallest extant even-toed ungulate. The weight of the Java mouse-deer ranges from , with males being heavier than females. It has an average tail length of about . Mouse-deer are thought to be the most primitive ruminants based on their behaviour and the fossil record, thus they are the living link between ruminants and non-ruminants.