These sharks possess the typical lamniform body plan and have been mistaken for great white sharks. The two species can be easily distinguished, however, by the basking shark's cavernous jaw (up to 1 m in width, held wide open whilst feeding), longer and more obvious gill slits (which nearly encircle the head and are accompanied by well-developed gill rakers), smaller eyes, and smaller average girth. Great white sharks possess large, dagger-like teeth, whilst those of the basking shark are much smaller (5–6 mm) and hooked; only the first 3 or 4 rows of the upper jaw and 6 or 7 rows of the lower jaw are functional. There are also several behavioural differences between the two (see Behaviour).
Other distinctive characteristics of the basking shark include a strongly keeled caudal peduncle, highly textured skin covered in placoid scales and a layer of mucus, a pointed snout (which is distinctly hooked in younger specimens), and a lunate caudal fin. In large individuals the dorsal fin may flop over when above the surface. Colouration is highly variable (and likely dependent on observation conditions and the condition of the animal itself): commonly, the colouring is dark brown to black or blue dorsally fading to a dull white ventrally. The sharks are often noticeably scarred, possibly through encounters with lampreys or cookiecutter sharks. The basking shark's liver, which may account for 25% of its body weight, runs the entire length of the abdominal cavity and is thought to play a role in buoyancy regulation and long-term energy storage.
In females, only the right ovary appears to be functional: if so, this is a unique characteristic among sharks.
Studies in 2003 have disproved the idea that basking sharks hibernate and have shown that they are active throughout the year. In winter, basking sharks move to deeper water (depths of up to 900 m) feeding on deep water plankton. Satellite tagging confirmed that basking sharks move thousands of kilometres during the winter months locating plankton blooms. It was also found that basking sharks shed and renew their gillrakers in an ongoing process, rather than over one short period.
They feed at or close to the surface with their mouths wide open and gill rakers erect. They are slow-moving sharks (feeding at about 2 knots) and do not attempt to evade approaching boats (unlike great white sharks). They are harmless to humans if left alone and will not be attracted to chum.
Basking sharks are social animals and form schools segregated by sex, usually in small numbers (3 or 4) but reportedly up to 100 individuals. Their social behaviour is thought to follow visual cues, as although the basking shark's eyes are small, they are fully developed and have been known to visually inspect boats, possibly mistaking them for conspecifics. Females are thought to seek out shallow water to give birth.
These sharks have few predators, but orcas and tiger sharks are known to feed on them, and the aforementioned lampreys are often seen attached to them, although it is unlikely that they are able to cut through the shark's thick skin.
The onset of maturity in basking sharks is not known but is thought to be between the age of 6 and 13 and at a length of between 4.6 and 6 m. Breeding frequency is also unknown, but is thought to be 2 to 4 years.
The seemingly useless teeth of basking sharks may play a role before birth since the basking shark seems to feed on unfertilized eggs in utero.
Historically, the basking shark has been a staple of fisheries because of its slow swimming speed, unaggressive nature and previously abundant numbers. Commercially it was put to many uses: the flesh for food and fishmeal, the hide for leather, and its large liver (which has a high squalene content) for oil. It is currently fished mainly for its fins (for shark fin soup). Parts (such as cartilage) are also used in traditional Chinese medicine and as an aphrodisiac in Japan, further adding to demand.
As a result of rapidly declining numbers, the basking shark has been protected and trade in its products restricted in many countries. It is fully protected in the UK, Malta, Florida and US Gulf and Atlantic waters. Targeted fishing for basking sharks is illegal in New Zealand. Once considered a nuisance along the Canadian Pacific coast, basking sharks were the target of a government eradication program there from 1945 to 1970. As of 2008, efforts are underway to determine if any sharks still live in the area and monitor their potential recovery.
It is tolerant of boats and divers approaching it and may even circle divers, making it an important draw for dive tourism in areas where it is common.