The story is set in Amity, a seaside resort town on Long Island, New York. One night, a massive great white shark kills a young tourist named Chrissie Watkins while she skinny dips in the open waters. After finding what remains of her body washed up on the beach, investigators realize she was attacked by a shark. Police chief Martin Brody orders Amity's beaches closed, but mayor Larry Vaughn and the town's selectmen overrule him out of fear for damage to summer tourism, the town's main industry. With the connivance of Harry Meadows, the editor of the local newspaper, they hush up the attack.
A few days later, the shark kills a young boy named Alex Kintner and an old man not far from the shore. A local fisherman, Ben Gardner, is sent by Amity's authorities to kill the shark but disappears on the water. Brody and deputy Leonard Hendricks find Gardner's boat anchored off-shore, empty, and covered with large bite holes, one of which has a massive shark tooth stuck in it. Blaming himself for these deaths, Brody again tries to close the beaches, while Meadows investigates the Mayor's business contacts to determine why he wants to keep the beaches open. Meadows uncovers links to the Mafia, who pressure Vaughn to keep the beaches open in order to protect the value of Amity's real estate, in which the Mafia invested a great deal of money. Meadows also recruits marine biologist Matt Hooper from the Woods Hole Institute to advise them on how to deal with the shark.
Meanwhile, Brody's wife Ellen misses the affluent life she had before marrying Brody and having children. She starts a romantic relationship with Hooper, who is the younger brother of a man she used to date, and the two have a brief affair in a motel outside of town. Throughout the rest of the novel, Brody suspects they have had a liaison and is haunted by the thought.
With the beaches still open, people pour to the town, hoping to glimpse the killer shark. Brody sets up patrols to watch for the fish. After a boy narrowly escapes another attack close to the shore, Brody closes the beaches and hires Quint, a professional shark hunter, to kill the shark. Brody, Quint, and Hooper set out on Quint's vessel, the Orca . The trio are soon at odds with one another. Quint's methods anger Hooper, especially when he disembowels a blue shark, and then uses an illegally fished unborn dolphin for bait. All the while, Quint taunts Hooper as a rich college boy. Brody and Hooper also argue, as Brody's suspicions about Hooper's possible affair with Ellen grow stronger; at one point, Brody unsuccessfully attempts to strangle Hooper.
Their first two days at sea are unproductive and they return to port each night. On the third day, Hooper wants to bring along a shark proof cage. Initially Quint refuses, considering it a suicidal idea, but he relents when Hooper offers him a hundred dollars. On the ocean, the shark finally shows up. Hooper guesses the animal must be at least twenty feet long, and is visibly excited and in awe. After several unsuccessful attempts by Quint to harpoon the shark, Hooper goes underwater in the cage. He resolves to first take photos of the shark, before trying to kill it with a bang stick. However, the shark attacks the cage, and, after ramming the bars apart, kills and eats Hooper.
Larry Vaughn arrives at the Brody house before Brody returns home, and informs Ellen that he and his wife are leaving Amity. Before he leaves, he tells Ellen that he always thought they would have made a great couple. When Brody and Quint return to sea the following day, the shark starts ramming the boat, but Quint is able to harpoon it several times. The shark leaps out of the water and onto the stern of the Orca , ripping a huge hole in the aft section and causing the boat to begin to sink. Quint plunges another harpoon into the shark, but as it falls back into the water, his foot gets entangled in the rope, and he is dragged underwater to his death as he drowns. Brody, now floating on a seat cushion, spots the shark swimming towards him and prepares for his death. However, just as the shark gets within a few feet of him, it succumbs to its many wounds, rolls over in the water, and dies before it can kill Brody. The great fish sinks down out of sight, dragging Quint's still entangled body behind it. Brody manages to paddle back to shore on his makeshift float, the lone survivor of the ordeal.