The largest living snakes in the world, measured either by length or by weight, are various members of the Boidae and Pythonidae families. They include anacondas, pythons and boa constrictors, which are all non-venomous constrictors. The longest venomous snake, with a length up to 18.5–18.8 ft (5.6–5.7 m), is the king cobra, while contesters for the heaviest title include the Gaboon viper and the Eastern diamondback rattlesnake. All of these three species reach a maximum mass in the range of 6–20 kg (13–44 lb).

There are fourteen or fifteen living snake species that clearly have a maximum mass of at least 50 lb (23 kg), as shown in the table below. Whether the number is fourteen or fifteen depends on whether a DNA analysis reported in 2024 results in the recognition of the northern green anaconda ("Eunectes akayima", listed in row 1b below) as a species distinct from the ordinary (southern) green anaconda (Eunectes murinus). These include all species that reach a length of at least 20 ft (6 m). There are also two other species that reach nearly this length – the Oenpelli python (binomial name Nyctophilopython oenpelliensis, Simalia oenpelliensis or Morelia oenpelliensis), and the olive python (Liasis olivaceus). The information available about these two species is rather limited. The Oenpelli python, in particular, has been called the rarest python in the world.

It is important to be aware that there is considerable variation in the maximum reported size of these species, and most measurements are not truly verifiable, so the sizes listed should not be considered definitive. In general, the reported lengths are likely to be somewhat overestimated. In spite of what has been, for many years, a standing offer of a large financial reward (initially $1,000 offered by U.S. President Theodore Roosevelt in the early 1900s, later raised to $5,000, then $15,000 in 1978 and $50,000 in 1980) for a live, healthy snake over 30 ft (9.14 m) long by the New York Zoological Society (later renamed as the Wildlife Conservation Society), no attempt to claim the reward has ever been made.

Although it is generally accepted that the reticulated python is the world's longest snake, most length estimates longer than 6 m (20 ft) have been called into question. It has been suggested that confident length records for the largest snakes must be established from a dead body soon after death, or alternatively from a heavily sedated snake, using a steel tape and in the presence of witnesses, and must be published (and preferably recorded on video). At least one reticulated python was measured under full anesthesia at 6.95 m (22.8 ft), and somewhat less reliable scientific reports up to 10.05 m (33.0 ft) have appeared.

Although weight is easier to measure reliably than length (e.g., by simply measuring the weight of a container with and without the snake inside it and subtracting one measurement from the other), a significant factor in the weight of a snake is whether it has been kept in captivity and provided an unusual abundance of food in conditions that also cause reduced levels of activity. Moreover, the weight of wild specimens is often reduced as a symptom of parasite infestations that are eliminated by veterinary care in captivity. Thus, the largest weights measured for captive specimens often greatly exceed the largest weights observed in the wild for the same species. This phenomenon may particularly affect the weight measurements for anaconda species that are especially difficult to keep in captivity due to their semi-aquatic nature, resulting in other species having larger weights measured in captivity. In particular, the green anaconda (Eunectes murinus) is an especially massive snake if only observations in the wild are considered.