A Connecticut Yankee in King Arthur's Court is a novel by American humorist and writer Mark Twain. The book was originally titled A Yankee in King Arthur's Court. Some early editions are titled A Yankee at the Court of King Arthur.

In the book, a Yankee engineer from Connecticut is accidentally transported back in time to the court of King Arthur, where he fools the inhabitants of that time into thinking that he is a magician, and soon uses his knowledge of modern technology to become a "magician" in earnest, stunning the English of the Early Middle Ages with such feats as demolitions, fireworks, and the shoring up of a holy well. He attempts to modernize the past, but in the end he is unable to prevent the death of Arthur and an interdict against him by the Catholic Church of the time, which grows fearful of his power.

Twain wrote the book as a burlesque of Romantic notions of chivalry after being inspired by a dream in which he was a knight himself, severely inconvenienced by the weight and cumbersome nature of his armor.

Twain first conceived of the idea behind A Connecticut Yankee in King Arthur's Court in December 1884 and worked on it between 1885 and 1889. It was first published in England by Chatto & Windus under the title A Yankee at the Court of King Arthur in December 1889. Writer and critic William Dean Howells called it Twain's best work and "an object-lesson in democracy".