A young woman named Chrissie Watkins leaves an evening beach party on Amity Island to go skinny dipping, only to be dragged back and forth and then pulled under the water by an unseen force. Amity's police chief, Martin Brody, is notified that Chrissie is missing, and Deputy Hendricks finds her remains on the beach. The medical examiner informs Brody that she was killed by a shark. Brody plans to close the beaches but is overruled by Mayor Larry Vaughan, who fears that reports of a shark attack will ruin the summer tourist season, the town's primary source of income and notes that the town has never had shark problems. The medical examiner consequently attributes the death to a boating accident. Brody reluctantly goes along with the explanation.
A short time later, a boy named Alex Kintner is killed by a shark at the beach. The boy's mother places a bounty on the shark, sparking an amateur shark-hunting frenzy and prompting local professional shark hunter Quint to offer his services for an even higher fee. Marine biologist Matt Hooper examines Chrissie's remains and determines that she was unquestionably killed by a shark.
A large tiger shark is caught by fishermen, leading the townspeople to believe the problem is solved, but Hooper is unconvinced and asks to examine its stomach contents. Vaughan refuses to make the autopsy public, so Brody and Hooper return after dark and discover the dead shark does not contain human remains. They come across the half-sunken wreckage of a boat belonging to local fisherman Ben Gardner. Hooper explores the vessel underwater and discovers a sizable shark's tooth protruding from the damaged hull before he is startled by Gardner's remains and drops the tooth. Due to lack of evidence, Vaughan refuses to close the beaches, and on the Fourth of July many tourists arrive. A practical joke by kids causes panic at the main beach while the shark enters a nearby estuary, killing a man; Brody's son, who witnesses the attack, goes into shock. Brody persuades Vaughan to hire Quint, who reluctantly allows Hooper to join the hunt along with Brody. The three set out to catch and kill the shark aboard Quint's vessel, the Orca.
Brody is given the task of laying a chum line while Quint uses fishing tackle to try to hook the shark. An enormous great white shark looms up behind the boat, and the trio watch it circle the Orca. Quint harpoons it with a line attached to a flotation barrel, but the shark pulls the barrel under and disappears.
By nightfall, the men retire to the boat's cabin, where Quint relates his experience with sharks as a survivor of the sinking of the warship USS Indianapolis during the War in the Pacific in 1945. The shark reappears, damaging the boat's hull before slipping away. In the morning, Brody attempts to call the Coast Guard, but Quint destroys the radio. After a long chase, Quint harpoons another barrel to the shark. The men tie it to the stern, but the shark drags the boat backward, forcing water onto the deck and flooding the engine. Quint heads toward shore, hoping to draw his quarry into shallow waters and suffocate it, but overtaxes and stalls the Orca's damaged engines.
With the boat immobilized, the trio attempt a desperate approach: Hooper dons scuba gear and enters the ocean inside a shark proof cage, attempting to stab the shark with a hypodermic spear filled with strychnine. However, the shark takes him by surprise, and Hooper drops his spear but manages to escape and hide on the seabed. As Quint and Brody raise the mangled cage, the shark leaps onto the boat, crushing the transom. Quint slips down the deck into the shark's mouth and is devoured. When the shark attacks Brody, he shoves a pressurized scuba tank into its mouth, then takes Quint's rifle and climbs the sinking Orca 's mast. Brody shoots at the scuba tank, blowing it and the shark to pieces which sinks to the bottom of the sea. Hooper emerges, and the two make rafts out of the Orca 's remains to paddle back to the shores of Amity Island.