Percy Jackson & the Olympians is a fantasy novel series by American author Rick Riordan. It is the first of four book series in the Camp Half-Blood Chronicles, preceding The Heroes of Olympus, The Trials of Apollo, and The Nico di Angelo Adventures. The novels are set in a world with the Greek gods in the 21st century. The series follows the protagonist, Percy Jackson, a young demigod who must prevent the Titans, led by Kronos, from destroying the world.

The first three books were published in the United States by Miramax Books before they were folded into Hyperion Books; that house published the remaining books. All the books were published in the United Kingdom by Penguin Books. Four supplementary books, along with graphic novel versions of the first five books, have also been released. By January 2022, the books had sold more than 180 million copies worldwide, making the series one of the best-selling of all time. A follow-up trilogy in the series, The Senior Year Adventures, takes place after the events of The Heroes of Olympus and began in 2023 with The Chalice of the Gods. Three sequel series, The Heroes of Olympus, The Trials of Apollo, and The Nico di Angelo Adventures, follow.

The first book was adapted as a film titled Percy Jackson & the Olympians: The Lightning Thief (2010). It was commercially successful but received mixed reviews from critics and the audience for its departure from the book. The first book was also adapted into a musical. The second book was adapted as a film titled Percy Jackson: Sea of Monsters (2013), which also received mixed reviews. A television series based on the novels premiered on Disney+ in 2023 to positive reviews.

Rick Riordan has said that the idea for the Percy Jackson series began when he was telling bedtime stories to his son, Haley Riordan. Haley had been diagnosed with ADHD and dyslexia, inspiring Riordan to make the titular protagonist, Percy Jackson, hyperactive and dyslexic.

Riordan created the fictional character of Percy Jackson and a story about his traveling across the United States to recover Zeus's lightning bolt. Haley suggested his dad should make it a book, and Riordan wrote what became known as The Lightning Thief over the next year.

After submitting his manuscript to his agent and editor, Riordan presented the book to a group of sixth, seventh, and eighth graders for their reactions. He gained their approval and, with their help, came up with the book title and the way Percy's sword works. In 2004, the book was sold to Miramax Books for enough money that Riordan could quit his job and focus on writing full-time. After it was published in 2005, it sold more than 1.2 million copies. The book was released in multiple editions, including hardcover, paperback, and audio editions. It has been translated into multiple languages and published internationally.

The original series is set in the mid-2000s and follows Perseus "Percy" Jackson, a 12-year-old boy who learns he is the son of the Greek god Poseidon. Zeus, Poseidon, and Hades had sworn not to father any more children after World War II, and as such, Poseidon did not interact with him as a child. The gods took that pledge because their children were too strong and had the potential to cause immense slaughter (in the universe's continuity, World War II was a conflict among their offspring). When Percy learns that he is at risk of being found by ferocious monsters because of his demigod status and strength, he also learns that Camp Half-Blood, a training facility on Long Island, New York, houses more demigods like him. He journeys with a new friend, Annabeth Chase, a daughter of Athena, and his best friend and traveling companion, Grover Underwood, a satyr who is his guardian. As the threat posed by the Titans grows, Percy begins to complete remarkable tasks, fulfill predictions, and engage in combat with and for the gods. He realizes that he can either contribute to the world's destruction or preservation.

The Lightning Thief is the first book in the series and was released on June 28, 2005.