Twenty Thousand Leagues Under the Seas: A Tour of the Underwater World (French: Vingt mille lieues sous les mers: Tour du monde sous-marin, "Twenty Thousand Leagues Under the Seas: A Tour of the Underwater World") is a classic science fiction adventure novel by French writer Jules Verne published in 1870.

The novel was originally serialized from March 1869 through June 1870 in Pierre-Jules Hetzel's periodical, the Magasin d'Éducation et de Récréation. The deluxe illustrated edition, published by Hetzel in November 1871, included 111 illustrations by Alphonse de Neuville and Édouard Riou. The book was highly acclaimed when it was released and still is; it is regarded as one of the premiere adventure novels and one of Verne's greatest works, along with Around the World in Eighty Days and Journey to the Center of the Earth. The description of Nemo's ship, the Nautilus, was considered ahead of its time, as it accurately describes features on submarines, which at the time were very primitive vessels.

The title refers to the distance traveled while under the sea and not to a depth, as 20,000 leagues (80,000 km) is nearly twice the circumference of the Earth. The greatest depth mentioned in the book is four leagues. The book uses metric leagues, which are four kilometres each.

During the year 1866, ships of several nations spot a mysterious sea monster, which some suggest to be a giant narwhal. The United States government assembles an expedition in New York City to find and destroy the monster. Professor Pierre Aronnax, a French marine biologist and narrator of the story, who happens to be in New York at the time, receives a last-minute invitation to join the expedition, which he accepts. Canadian whaler and master harpoonist Ned Land and Aronnax's faithful servant Conseil are also brought aboard.

The expedition departs Brooklyn aboard the United States Navy frigate Abraham Lincoln and travels south around Cape Horn into the Pacific Ocean. The ship finds the monster after a long search and then attacks the beast, which damages the ship's rudder. The three protagonists are then hurled into the water and grasp hold of the "hide" of the creature, which they find, to their surprise, to be a submarine very far ahead of its era. They are quickly captured and brought inside the vessel, where they meet its enigmatic creator and commander, Captain Nemo.