The frilled lizard (Chlamydosaurus kingii), also known commonly as the frilled agama, frillneck lizard, frill-necked lizard or frilled dragon, is a species of lizard in the family Agamidae. The species is native to northern Australia and southern New Guinea and is the only member of the genus Chlamydosaurus. Its common names refer to the large frill around its neck, which usually stays folded against the lizard's body. The frilled lizard grows to 90 cm (35 in) from head to tail tip and can weigh 600 g (1.3 lb). Males are larger and more robust than females. The lizard's body is generally grey, brown, orangish-brown, or black in colour. The frills have red, orange, yellow, or white colours.

The frilled lizard is largely arboreal, spending most of its time in trees. Its diet consists mainly of insects and other invertebrates. It is more active during the wet season, when it spends more time near or on the ground, and is less observed during the dry season, during which it seeks shade in the branches of the upper canopy. It breeds in the late dry season and early wet season. The lizard uses its frill to scare off predators and display to other individuals. The species is considered to be of least concern by the International Union for Conservation of Nature.

British zoologist John Edward Gray described the frilled lizard in 1825 as Clamydosaurus kingii. He used a specimen collected by botanist Allan Cunningham at Careening Bay, off north-western Australia, while part of an expedition conducted by Captain Phillip Parker King in HMS Mermaid. The generic name, Chlamydosaurus, is derived from the Ancient Greek chlamydo (χλαμύς), meaning "cloaked" or "mantled", and Latin saurus (sauros), meaning "lizard". The specific name, kingii, is a Latinised form of King. It is the only species classified in its genus.

The frilled lizard is classified in the family Agamidae and the subfamily Amphibolurinae. It split from its closest living relatives around 10 million years ago based on genetic evidence. A 2017 mitochondrial DNA analysis of the species across its range revealed three lineages demarcated by the Ord River and the southeast corner of the Gulf of Carpentaria (Carpentarian Gap). One lineage ranged across Queensland and southern New Guinea and is sister to one that ranged from western Queensland to the Ord River. The ancestor of these two split from a lineage that populates the Kimberley. Frilled lizards entered southern New Guinea possibly around 17,000 years ago during a glacial cycle, when sea levels were lower and a land bridge connected the island to Cape York. The study upholds C. kingii as one species with the different populations being "shallow allopatric clades".

The following cladogram is based on Pyron and colleagues (2013).

The frilled lizard can reach a total length of around 90 centimetres (35 in) and a head-body length of 27 cm (11 in), and weighs at least 600 g (1.3 lb). It has a particularly large and wide head; a long neck to accommodate the frill; long legs and a tail that makes most of its total length. The species is sexually dimorphic, males being larger than females and having proportionally bigger frills, heads and jaws. The corners of the frilled lizard's eyes are pointed and the rounded nostrils face away from each other and angle downward. Most of the lizard's scales are keeled, having a ridge down the centre. From the backbone to the sides, the scales alternate between small and large.

The distinctive frill is a flap of skin that extends from the head and neck and contains several folded ridges. When fully extended, the frill is disc-shaped and can reach over four times the length of the animal's torso in diameter, or around 30 cm (12 in) across. When not extended, the frill wraps around the body, like a cape over the neck and shoulders. The frill is laterally symmetrical; the right and left sides are attached at the bottom in a V-shape, and cartilage-like connective tissue (Grey's cartilage) connects the top ends to each side of the head near the ear openings. The frill is supported by rod-like hyoid bones, and is spread out by movements of these bones, the lower jaw and Grey's cartilage. This structure mainly functions as a threat display to predators and for communication between individuals. It can also act as camouflage when folded, but this is unlikely to have been a consequence of selection pressure. The frill may be capable of working like a directional microphone, allowing them to better hear sounds directly in front of them but not around them. There is no evidence for other suggested functions, such as food storage, gliding or temperature regulation.

Frilled lizards vary between grey, brown, orangish-brown, and black dorsally, the underside being paler white or yellow. Males have a dark belly but a lighter chest. The underside and lateral sides of the species are sprinkled with dark brown markings that merge to create bands on the tail. The colours of the frills vary based on range; lizards west of the Ord River have red-coloured frills, those living between the river and the Carpentarian Gap have orange frills, and those east of the gap have yellow to white frills. New Guinean frilled lizards are yellow-frilled. The more colourful frills have white patches which may add to the display. Colouration is mainly created by carotenoids and pteridine pigments; lizards with red and orange frills have more carotenoids than those with yellow and white frills, the latter two are also lacking in pteridines. Yellow colouration has been linked to higher steroid hormones.