"Ali Baba and the Forty Thieves" (Arabic: علي بابا والأربعون لصا) is a folk tale in Arabic added to the One Thousand and One Nights in the 18th century by its French translator Antoine Galland, who heard it from Syrian storyteller Hanna Diyab. As one of the most popular Arabian Nights tales, it has been widely retold and performed in many media across the world, especially for children (for whom the more violent aspects of the story are often removed).

In the original version, Ali Baba (Arabic: عَلِيّ بَابَا, romanized: ʿAliyy Bābā) is a poor woodcutter and an honest person who discovers the secret treasure of a thieves' den, and enters with the magic phrase "open sesame". The thieves try to kill Ali Baba while his rich and greedy brother Cassim (Arabic: قاسم Qāsim, sometimes spelled 'Kasim' or 'Qasim') tries to steal the treasure for himself, but Ali Baba’s faithful slave-girl Morgiana (Arabic: مرجانة Murjāna) foils their plots. His son marries her, and Ali Baba keeps the secret of the treasure.

The tale was added to the story collection One Thousand and One Nights by one of its European translators, Antoine Galland, who called his volumes Les Mille et Une Nuits (1704–1717). Galland was an 18th-century French Orientalist who heard it in oral form from a Syrian Maronite story-teller called Hanna Diyab, who came from Aleppo in modern-day Syria and told the story in Paris. In any case, the earliest known text of the story is Galland's French version. Richard F. Burton included it in the supplemental volumes (rather than the main collection of stories) of his translation (published as The Book of the Thousand Nights and a Night).

The American Orientalist Duncan Black MacDonald discovered an Arabic-language manuscript of the story at the Bodleian Library; however, this was later found to be a counterfeit.

Ali Baba and his older brother Cassim are the sons of a merchant. After their father's death, the greedy Cassim marries a wealthy woman and becomes well-to-do, living lazily on their father's business and his wife's wealth. Ali Baba marries a poor woman and settles into the trade of a woodcutter. Cassim and his wife resent Ali Baba and his side of the family and do not share their wealth with them.

One day, Ali Baba is at work collecting and cutting firewood in the forest, when he happens to overhear a group of forty thieves visiting their stored treasure. The treasure is in a cave, the mouth of which is sealed by a huge rock. It opens on the magic words "open sesame" and seals itself on the words "close sesame". When the thieves are gone, Ali Baba enters the cave himself and although there is a vast amount of riches stashed inside, he modestly takes only a single bag of gold coins home.

Ali Baba and his wife borrow his sister-in-law's scales to weigh their new wealth. Unbeknownst to them, Cassim's wife puts a blob of wax in the scales to find out what Ali Baba is using them for, as she is curious to know what kind of grain her impoverished brother-in-law needs to measure.

To her shock, she finds a gold coin sticking to the scales and tells her husband. Under pressure from his brother, Ali Baba is forced to reveal the secret of the cave. Cassim goes to the cave, taking a donkey with him to take as much treasure as possible. He enters the cave with the magic words. However, in his excited greed over the treasure, he forgets the words to get out again and ends up trapped. The thieves find him there and kill him. When his brother does not come back, Ali Baba goes to the cave to look for him, and finds the body quartered and with each piece displayed just inside the cave's entrance, as a warning to anyone else who might try to enter.