The Greenland shark (Somniosus microcephalus), also known as the gurry shark, grey shark, or by the Kalaallisut name eqalussuaq, is a large shark of the family Somniosidae ("sleeper sharks"), closely related to the Pacific and southern sleeper sharks. The distribution of this species is mostly restricted to the waters of the North Atlantic Ocean and Arctic Ocean.

Greenland sharks have the longest known lifespan of all vertebrate species (estimated to be between 300 - 500 years), and the species is among the largest extant species of shark. As an adaptation to living at depth, it has a high concentration of trimethylamine N-oxide in its tissues, which causes the meat to be toxic] Greenland shark flesh treated to reduce toxin levels is eaten in Iceland as a delicacy known as kæstur hákarl.

The Greenland shark is an apex predator and mostly eats fish. It has never been observed hunting. Recorded fish prey have included smaller sharks, skates, eels, herring, capelin, Arctic char, cod, rosefish, sculpins, lumpfish, wolffish, and flounder.

Greenland sharks have also been found with remains of seals, polar bears, moose, and reindeer (in one case an entire reindeer body) in their stomachs. The Greenland shark is known to be a scavenger, and is attracted by the smell of rotting meat in the water. The sharks have frequently been observed gathering around fishing boats It also scavenges on seals.

Although such a large shark could easily consume a human swimmer, the frigid waters it typically inhabits make the likelihood of attacks on humans very low, and no cases of predation on people have been verified.

The flesh of the Greenland shark is toxic because of the presence of high concentrations of trimethylamine oxide (TMAO). If the meat is eaten without pretreatment, the ingested TMAO is metabolized into trimethylamine, which can produce effects similar to extreme drunkenness. Occasionally, sled dogs that eat the flesh are unable to stand up because of this effect. Similar toxic effects occur with the related Pacific sleeper shark, but not in most other shark species.

The meat can be treated for safe consumption by boiling in several changes of water, drying, or fermenting for several months to produce kæstur hákarl. Traditionally, this is done by burying the meat in boreal ground for six to eight weeks, which presses the TMAO out of the meat and also results in partial fermentation. The meat is then dug up and hung up in strips to dry for several more months. It is considered a delicacy in Iceland.

The Greenland shark's poisonous flesh has a high urea content, which gave rise to the Inuit legend of Skalugsuak, the first Greenland shark. The legend says that an old woman washed her hair in urine and dried it with a cloth. The cloth blew into the ocean to become Skalugsuak. Another legend tells of a girl whose father cut off her fingers while drowning her, with each finger turning into a sea creature, including Skalugsuak.

The Greenland shark plays a role in cosmologies of the Inuit from the Canadian Eastern Arctic and Greenland. Igloolik Inuit believe that the shark lives within the urine pot of Sedna, goddess of the sea, and consequently its flesh has a urine-like smell, and acts as a helping spirit to shamans.