Seahorses are mainly found in shallow tropical and temperate waters throughout the world, and live in sheltered areas such as seagrass beds, estuaries, coral reefs, or mangroves. Four species are found in Pacific waters from North America to South America. In the Atlantic, H. erectus ranges from Nova Scotia to Uruguay. The dwarf seahorse, is found in the Bahamas.

Colonies have been found in European waters such as the Thames Estuary. Three species live in the Mediterranean Sea: the long-snouted seahorse, the short-snouted seahorse, and the sea pony. These species form territories; males stay within about 11 square feet of habitat, while females range about one hundred times that amount.

Spiny seahorses from East Timor typically hold onto soft coral with their prehensile tail. Seahorses range in size from 1.5 to 35.5 cm. They are named for their equine appearance. Although they are bony fish, they do not have scales, but rather thin skin stretched over a series of bony plates, which are arranged in rings throughout their bodies. Each species has a distinct number of rings. Seahorses swim upright, another characteristic not shared by their close pipefish relatives, which swim horizontally. Razorfish are the only other fish that swim vertically like a seahorse. Unusual among fish, a seahorse has a flexible, well-defined neck. It also sports a coronet on its head, which is distinct for each individual.

Seahorses swim very poorly, rapidly fluttering a dorsal fin and using pectoral fins (located behind their eyes) to steer. The slowest-moving fish in the world is the dwarf seahorse, with a top speed of about 5 ft per hour. Since they are poor swimmers, they are most likely to be found resting with their prehensile tails wound around a stationary object. They have long snouts, which they use to suck up food, and their eyes can move independently of each other like those of a chameleon.