The Hidden Oracle is a fantasy novel based on Greek and Roman mythology written by American author Rick Riordan. It was published on May 3, 2016, and is the first book in The Trials of Apollo series, the second spin-off of the Percy Jackson & the Olympians series. The book and its cover art by John Rocco were first announced in 2015. It has been published in hardcover, audiobook, ebook, and large-print editions. The Hidden Oracle has been translated into 19 languages from its original English.

The book follows the god Apollo, who is turned into a human teenager and thrown down from Olympus to New York City as a punishment by his father, Zeus. Joined by the demigod Meg McCaffrey, Apollo goes to Camp Half-Blood, where he discovers that he will have to regain control of the five oracles of Ancient Greece in order to receive pardon from Zeus.

The novel received positive reviews from critics, who praised Apollo's narration and the book's humor. During its first week of release, The Hidden Oracle sold about 62,000 copies, reaching the top of The New York Times and Publishers Weekly bestseller lists. It won the 2016 Goodreads Choice Award for Middle Grade and Children's Fiction.

After falling into a dumpster in a New York City alley, the god Apollo has only very vague memories of his father, Zeus, punishing him. He learns that he was turned into a human teenager named Lester Papadopoulos. In the alley, two thugs try to mug him, but a young girl named Meg McCaffrey saves him, using fruit to chase the thugs away. Meg claims Apollo's service, binding him to her until he is done with his trials, which he must complete to become a god again. Apollo and Meg go to find Percy Jackson.

With the help of Percy, and his mother, Sally Jackson, Apollo and Meg journey to Camp Half Blood, a camp for demigods. On their way, they get attacked by plague spirits. Meg subconsciously summons a karpos, who defeats the spirits. Meg decides to keep him and names him Peaches, but he only shows up when she's in trouble. After arriving at the Camp, Apollo discovers that the Oracle of Delphi, in the form of Rachel Elizabeth Dare, can no longer issue prophecies; similarly, travel and communication do not work for any of the demigods. The centaur Chiron also mentions that campers have been randomly disappearing into the woods.

At dinner, Meg is attacked by demigods who she previously angered. Peaches comes to her rescue, but the other demigods then attack Peaches, believing him to be dangerous. To save him, Meg reveals that her golden rings can turn into sickles made of Imperial gold. Demeter, the goddess of agriculture, claims her as her daughter.

The next day, Apollo tries to practice music and archery; being imperfect, he swears on the river Styx to never use a bow or a musical instrument until he is a god again. During a "three-legged death race" inside the Labyrinth, Apollo and Meg end up under Delphi in Greece. They overhear Delphi's guard, Python, talking to "the Beast" about how to control all the oracles and destroy the Grove of Dodona. “The Beast” assures Python that he has “well-placed help within the camp.” The encounter terrifies them, especially Meg. They escape the Labyrinth, and Chiron reveals that Apollo's children, Kayla and Austin, disappeared. Chiron and Meg advise Apollo to not search for them and the other missing demigods just yet, much to his anger. Instead, Apollo and Meg keep a lookout for them, and Meg reveals that she knows “the Beast” because of his reputation of taking demigods to train and use as servants. When Meg refused to work for him, he killed her father. Her stepfather then took her in, gave her the swords, and taught her how to fight.

Rachel arrives at the camp. She reveals that a secretive company, Triumvirate Holdings, has conspired against the gods and is attempting to control all the oracles, starting with Dodona, which is located at the camp and has been drawing campers to itself. The next day, Apollo and Meg go searching for the grove, but are attacked by myrmekes. In an attempt to drive them away, Apollo plays music, breaking his oath. The myrmekes kidnap Meg and escape. Apollo tries to return to the camp, but begins to hallucinate; before passing out, he finds Rhea, who gives him wind chimes to put on the largest tree in the Grove of Dodona and teleports him back to camp. Upon awakening, he learns that the leader of Triumvirate Holdings is Emperor Nero.