Megaherbivores (Greek μέγας megas "large" and Latin herbivora "herbivore") are large herbivores that can exceed 1,000 kg (2,200 lb) in weight. The earliest herbivores to reach such sizes like the pareiasaurs appeared in the Permian period. During most of the Mesozoic, the megaherbivore niche was largely dominated by dinosaurs up until their extinction during the Cretaceous–Paleogene extinction event. After this period, small mammalian species evolved into large herbivores in the Paleogene. As part of the Late Pleistocene megafauna extinctions, 80% of megaherbivore species became extinct, with megaherbivores becoming entirely extinct in Europe, Australia and the Americas. Recent megaherbivores include elephants, rhinos, hippos, and giraffes. There are nine extant species of terrestrial megaherbivores living in Africa and Asia. The African bush elephant is the largest extant species.

Extant megaherbivores are keystone species in their environment. They defoliate the landscape and spread a greater number of seeds than other frugivores. Extant megaherbivores, like most large mammals, are K-selected species and are characterized by their large size, relative immunity to predation, their effect on plant species, and their dietary tolerance.

Megaherbivores are large herbivores that weigh more than 1 ton when fully grown. They include both terrestrial and marine (i.e., manatees and dugongs) herbivores, and are classified as the largest type of (herbivorous) terrestrial megafauna (>45 kg).

Megaherbivores first evolved in the early Permian (300 mya). The earliest megaherbivores were synapsids; they became somewhat rare after the Permian–Triassic extinction event. Taxa mainly consisted of dicynodonts, and pareiasaurs. The exact cause of the extinction remains unknown. It is thought that the main cause of extinction was the flood basalt volcanic eruptions that created the Siberian Traps, which released sulfur dioxide and carbon dioxide, resulting in euxinia, elevating global temperatures, and acidifying the oceans.

Lisowicia was the last dicynodont that lived and became extinct in the Late Triassic. Some scientists have proposed that there was never a Triassic–Jurassic extinction event, but others argue that the extinctions occurred earlier. Nevertheless, flood basalts are thought to be the primary driver of the extinctions towards the end of the Triassic.

The taxonomic structure then switched to sauropodomorphs. Other taxa included stegosaurs and ankylosaurs. The change in taxonomy approximately occurred at the same time with the divergence of predominant vegetation and with extinctions. New taxa may have caused competitive exclusion (i.e. predominating and removing another taxa), or they may have adopted the ecological niche of extinct groups.

From the Triassic to the Cretaceous, a diverse assemblage of megaherbivorous dinosaurs, such as sauropods, occupied different ecological niches. Based on their dentition, ankylosaurs may have mainly consumed succulent plants, as opposed to nodosaurs, which were mainly browsers. It is thought that ceratopsids fed on rugged vegetation, due to their jaw being designed for a crushing effect. Studies on hadrosaur dentition concluded that they primarily fed on fruits.

Following the Cretaceous–Paleogene mass extinction, megaherbivore dinosaurs were extirpated from the face of the earth. One mechanism is thought to have played a major role: an extraterrestrial impact event in the Yucatán Peninsula. For about 25 million years, the earth was void of large terrestrial herbivores that weighed more than 1 ton. After this period, small mammalian species evolved into large herbivores across every continent around 40 mya. The largest of these animals were Paraceratheriidae and Proboscidea. Other taxa included Brontotheriidae. The Sirenia, aquatic megaherbivores, such as Dugongidae, Protosirenidae, and Prorastomidae were present in the Eocene. Megaherbivores inhabited every major landmass in the Cenozoic and Pleistocene before the arrival of humans.