Reports without physical proof are considered dubious if from nonscientists, as such individuals may at worst be more interested in promoting themselves or telling a good tale, or at the least may not be sufficiently trained in proper measurement methods. Skins can stretch substantially, increasing the snake's size by more than 50% if stretched during the tanning process. Transporting very large specimens to museums, especially before substantial decay, is difficult (though this has not prevented the return of much larger and more cumbersome crocodilian specimens). The remote location of the snake's habitat has historically made locating, capturing, and returning specimens difficult.

The eyes are set high on the head, allowing the snake to see out of the water while swimming without exposing its body. The head is narrow compared to the body, usually with distinctive orange-yellow striping on either side. The color pattern consists of olive green background overlaid with black blotches along the length of the body. Jesús Antonio Rivas, who had examined thousands of anacondas, was a female measuring 521 cm (17.09 ft) long and weighing 97.5 kg (215 lb). The longest (and heaviest) verified specimen encountered by Dr. A $50,000 cash reward is offered for anyone who can catch an anaconda 30 ft (9.1 m) or longer, but the prize has not been claimed yet. Reports of anacondas 35–40 feet or even longer also exist, but such claims need to be regarded with caution, as no specimens of such lengths have ever been deposited in a museum and hard evidence is lacking. Eunectes murinus is probably the heaviest extant species of snake or squamate in the world, perhaps only rivaled by the Komodo dragon. Although it is slightly shorter than the Reticulated python, it is far more robust: the bulk of a 4.5m green anaconda would be comparable to a 7.4m reticulated python. It is the largest snake native to the Americas. Weights are less well studied, though will reportedly range from 30 to 70 kg (66 to 154 lb) in an average-range adult. More typical mature specimens reportedly can range up 5 m (16.4 ft), with the females, at around a mean length of 4.6 m (15.1 ft), being generally much larger in adulthood than the male, which averages around 3 m (9.8 ft). The green anaconda is the world's heaviest and one of the world's longest snakes, reaching 5.21 m (17.1 ft) long. Other common names include common anaconda and water boa.

The term anaconda (without further qualification) often refers to this species, though the term could also apply to other members of the genus Eunectes. It is the largest, heaviest, and one of the longest known extant snake species.

Eunectes murinus (derived from the Greek ευνήκτης meaning 'good swimmer' and the Latin murinus meaning 'of mice' for being thought to prey on mice), commonly known as the green anaconda, is a non- venomous boa species found in South America.

Green anaconda