Twenty Thousand Leagues Under the Sea (French: Vingt mille lieues sous les mers) is a classic science fiction novel by French writer Jules Verne published in 1870. It tells the story of Captain Nemo and his submarine Nautilus as seen from the perspective of Professor Pierre Aronnax. The original edition had no illustrations; the first illustrated edition was published by Hetzel with illustrations by Alphonse de Neuville and Édouard Riou.

The title refers to the distance traveled while under the sea and not to a depth, as twenty thousand leagues is over six times the diameter of Earth. The greatest depth mentioned in the book is four leagues. In the book, a league is equivalent to four kilometres. A literal translation of the French title would end in the plural "seas", thus implying the "seven seas" through which the characters of the novel travel; however, the early English translations of the title used "sea", meaning the ocean in general.