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Terrestrial animals are animals that live predominantly or entirely on land, as compared with aquatic animals, which live predominantly or entirely in the water (e.g., fish, lobsters, octopuses), or amphibians, which rely on a combination of aquatic and terrestrial habitats (e.g., frogs or some crabs). Terrestrial animals evolved from marine animals (aquatic animals living in the ocean).

Terrestrial animals do not form a unified clade, rather they share only the fact that they are animals which live on land. There are 33 phyla^[1] of animals, of which 11 (33%) contain species which are found in terrestrial environments. Thus terrestrial animals span the widest range of the entire animal clade. The transition from an aquatic to terrestrial life has evolved independently and successfully many times by various animals.

Three of the phyla, Platyhelmithes (flatworms), Acanthocephala (hookworms), and Pentastoma (tongue worms), are found strictly as internal parasites of other terrestrial animals and so may not be considered truly terrestrial. Three of the phyla, Nemotoda (nematodes), nematomorpha (horsehair worms), and Tardigrada (water bears) are free living but need a constant external source of moisture. Five of the phyla, mollusks (snails and slugs), annelida (earthworms), Onychophors (velvet worms), Arthropoda, (woodlice, spiders, insects, crabs, and others), Chordata (tetrapods) are able to survive independently—they do not need constant external moisture, and can live outside the bodies of other terrestrial animals.

Classifying an animal species as terrestrial is often a matter of disputed judgment.

Many animals which are universally considered terrestrial do not have a life-cycle that is independent of water. Many insects and all terrestrial crabs (as well as other clades) have an aquatic life cycle stage. Their eggs need to be laid and to hatch in water. After hatching there is an early aquatic form, either a nymph or larva.

Crabs are of particular interest. There are crab species which are completely aquatic, crab species which are amphibious, and crab species which are terrestrial. The boundaries between these groups contain borderline species that are difficult to classify. There are no universally accepted criteria for deciding which life style to classify these borderline species.

The fiddler crabs are called “semi-terrestrial” since they make burrows in the muddy substrate to which they retreat during high tides. When the tide is out, fiddler crabs tirelessly scurry sideways along the beach as they comb the sands for food. In a mirror reversal of the fiddler crab life style penguins, seals and walruses sleep on land and feed in the ocean, yet they are all considered terrestrial. In all of these cases the life style of the ancestral species, and of the other species in the clade which are currently extant influences the designation. Thus a crab is more likely to receive an aquatic designation than is a seal, even though they spend the same amount of time on land and in the water.