The saltwater crocodile, also known as the estuarine or Indo-Pacific crocodile, is the largest of all living reptiles. This is a formidable, opportunistic and adaptable predator which occurs over a considerable range. It is found in suitable habitats from Northern Australia through Southeast Asia to the eastern coast of India, historically ranging as far west as off the eastern coast of Africa and as far east as waters off of Japan.

In northern Australia (which includes the northernmost parts of the Northern Territory, Western Australia and Queensland) the saltwater crocodile is thriving, particularly in the multiple river systems near Darwin (such as the Adelaide, Mary and Daly Rivers, along with their adjacent billabongs and estuaries) where large individuals of more than 5 metres (16 ft) in length are not uncommon. The saltwater crocodile population in Australia is estimated at somewhere between 100,000 and 200,000 adults. In Australia, the species co-exists with the smaller, narrow-snouted Johnston's or freshwater crocodile. Their range extends from Broome in Western Australia through the entire Northern Territory coast all the way down to Rockhampton in Queensland. The Alligator Rivers in the Arnhem Land region are misnamed due to the resemblance of the saltwater crocodile to alligators as compared to freshwater crocodiles, which also inhabit the Northern Territory.

Occasionally, saltwater crocodiles will attack and kill humans, although conflicts are generally one-sided in favour of humans, as this crocodilian has a highly valued hide.

The saltwater crocodile has fewer armour plates on its neck than other crocodilians. On this species, a pair of ridges run from the eyes along the centre of the snout. The scales are oval in shape and the scutes are small compared to other species. The adult saltwater crocodile's broad body contrasts with that of most other lean crocodiles, leading to early unverified assumptions the reptile was an alligator. The head is very large. Skull lengths more than 75 centimetres (30 in) have been confirmed for the species and mandibular length has been reported up to 98.3 centimetres (38.7 in) (female skull lengths of over 50 centimetres (20 in) are exceptional). The teeth are also long, with the largest teeth (the fourth tooth from the front on the lower jaw) having been measured to 9 centimetres (3.5 in) in length. If detached from the body, the head of a very large male crocodile can reportedly scale over 200 kg (440 lb) alone.

Young saltwater crocodiles are pale yellow in colour with black stripes and spots on the body and tail. This colouration lasts for several years until the crocodile matures into an adult. The colour as an adult is much darker greenish-drab, with a few lighter tan or grey areas sometimes apparent. Several colour variations are known and some adults may retain fairly pale skin whereas others may be so dark as to appear blackish. The ventral surface is white or yellow in colour on crocodiles of all ages. Stripes are present on the lower sides of the body but do not extend onto the belly. The tail is grey with dark bands.

Generally very lethargic – a trait which helps it survive months at a time without food – the saltwater crocodile typically loiters in the water or basks in the sun through much of the day, preferring to hunt at night. A study of seasonal saltwater crocodile behaviour in Australia indicates they are more active and more likely to spend more time on land in the Australian summer and less active and spend relatively more time basking in the sun during the Australian winter.

The saltwater crocodile is an opportunistic apex predator capable of taking nearly any animal that enters its territory, either in the water or on dry land. Like most crocodilians, they are unpicky eaters who readily vary their prey selection based on availability but are not voracious eaters, as they are able to survive on relatively little food for a prolonged period. The saltwater crocodile may take animals of almost any variety as it becomes available to them and, due to the enormous power and size of the species, it may take the broadest of prey species of any modern crocodilian. Juveniles are restricted to feeding on smaller animals such as insects, amphibians, crustaceans, small reptiles, and fish. The larger the animal grows, the greater the variety of animals it includes in its diet, although relatively small aquatic prey (especially fish) make up an important part of the diet even in adults. Large adult saltwater crocodiles can potentially eat any animal within their range. Wild animals taken by adult crocodiles can range from small to large and formidable, including monkeys, kangaroos, wild boar, dingos, snakes, turtles, goannas, other lizards, amphibians, water buffalo, gaurs and even sharks Humans and any type of domestic livestock or pet may be taken opportunistically as prey. Ground-living birds (including emus) and any type of water bird may be preyed upon. Even swift flying birds and bats may be snatched if they come close to the surface of water. They are dominant over other crocodilians, regularly outcompeting and occasionally killing and eating other species, as has been recorded largely with freshwater crocodiles in Australia. Domestic cattle, horses, water buffalo, and gaur, all of which may weigh over a ton, are considered the largest prey taken by male crocodiles. Perhaps the Asian elephant and the Asian rhinoceros species are the only non-marine animal in this species' range that this crocodile has not been known to predate.

Saltwater crocodiles can swim at 15 to 18 miles per hour (6.7 to 8.0 m/s) in short bursts, around three times as fast as the fastest human swimmers, but when cruising they usually go at 2 to 3 mph (0.9 to 1.3 m/s). Saltwater crocodiles are capable of explosive bursts of speed when launching an attack from the water, but stories of crocodiles being faster than a race horse for short distances across land are little more than urban legend. At the water's edge, however, where they can combine propulsion from both feet and tail, their speed can be considerable, though eyewitness accounts are rare.