Sea snake, any of more than 60 species of highly venomous marine snakes of the cobra family (Elapidae). There are two independently evolved groups: the true sea snakes (subfamily Hydrophiinae), which are related to Australian terrestrial elapids, and the sea kraits (subfamily Laticaudinae), which are related to the Asian cobras. 

Although their venom is the most potent of all snakes, human fatalities are rare because sea snakes are not aggressive, their venom output is small, and their fangs are very short.

Of the 55 species of true sea snakes, most adults are 1–1.5 metres (3.3–5 feet) long, though some individuals may attain 2.7 metres. They are restricted to coastal areas of the Indian and western Pacific oceans, except for the yellow-bellied sea snake (Pelamis platurus), found in open ocean from Africa eastward across the Pacific to the west coast of the Americas. All other species live mainly in waters less than 30 metres (100 feet) deep, as they must dive to the seafloor to obtain their food among coral reefs, among mangroves, or on the ocean bottom. Some species prefer hard bottoms (corals), while others prefer soft bottoms (mud or sand) in which to hunt their prey. Most sea snakes feed upon fishes of various sizes and shapes, including eels. Two primitive groups (genera Aipysurus and Emydocephalus) eat only fish eggs; Hydrophis specializes in burrowing eels.

In adaptation to marine life, true sea snakes have a flattened body with a short oarlike tail, valvular nostrils on top of the snout, and elongated lungs that extend the entire length of the body. Their scales are very small and usually not overlapping (juxtaposed), abutting against one another like paving stones. The belly scales are reduced in size in the primitive species, whereas in the more advanced forms they are absent. As a result, the advanced species cannot crawl and are thus helpless on land. When swimming, a keel is formed along part of the belly, increasing surface area and aiding propulsion, which occurs by lateral undulation. Sea snakes can remain submerged for several hours, possibly as much as eight or more. This remarkable feat is partly due to the fact that they can breathe through their skin. 

More than 90 percent of waste carbon dioxide and 33 percent of their oxygen requirement can be transported via cutaneous respiration. Sea snakes give birth in the ocean to an average of 2–9 young, but as many as 34 may be born. The 54 species in subfamily Hydrophiinae belong to 16 different genera.

The six species of sea kraits (genus Laticauda) are not as specialized for aquatic life as the true sea snakes. Although the tail is flattened, the body is cylindrical, and the nostrils are lateral. They have enlarged belly scales like those of terrestrial snakes and can crawl and climb on land. The typical colour pattern consists of alternating bands of black with gray, blue, or white rings. The yellow-lipped sea krait (L. colubrina) is a common species that possesses this pattern and has a yellow snout. 

Sea kraits are nocturnal, feeding primarily on eels at depths of less than 15 metres. They go ashore to lay their eggs, climbing up into limestone caves and rock crevices, where they deposit 1–10 eggs. Adults average 1 metre in length, but some grow to more than 1.5 metres. The longevity record in captivity is seven years.