The Moonstone (1868) by Wilkie Collins is a 19th-century British epistolary novel. It is generally considered to be the first detective novel, and it established many of the ground rules of the modern detective novel. The story was originally serialised in Charles Dickens' magazine All the Year Round. The Moonstone and The Woman in White are widely considered to be Collins' best novels, and Collins adapted The Moonstone for the stage in 1877, although the production was performed for only two months.

The Moonstone of the title is a diamond, not to be confused with the semi-precious moonstone gem. It gained its name from its association with the Hindu god of the moon Chandra. It was said to be protected by hereditary guardians on the orders of Vishnu, and to wax and wane in brilliance along with the light of the moon.

Rachel Verinder, a young English woman, inherits a large Indian diamond on her eighteenth birthday. It is a legacy from her uncle, a corrupt British army officer who served in India. The diamond is of great religious significance as well as being extremely valuable, and three Hindu priests have dedicated their lives to recovering it. The story incorporates elements of the legendary origins of the Hope Diamond (or perhaps the Orloff Diamond or the Koh-i-Noor diamond). Rachel's eighteenth birthday is celebrated with a large party, at which the guests include her cousin Franklin Blake. She wears the Moonstone on her dress that evening for all to see, including some Indian jugglers who have called at the house. Later that night, the diamond is stolen from Rachel's bedroom, and a period of turmoil, unhappiness, misunderstandings and ill-luck ensues. Told by a series of narratives from some of the main characters, the complex plot traces the subsequent efforts to explain the theft, identify the thief, trace the stone and recover it.