Nadia: The Secret of Blue Water (ふしぎの海のナディア, Fushigi no Umi no Nadia, lit. 'Nadia of the Mysterious Seas') is a Japanese anime television series created by NHK, Toho, and Korad, from a concept of Hayao Miyazaki, and directed by Hideaki Anno of Gainax. Inspired by the works of Jules Verne, particularly Twenty Thousand Leagues Under the Seas and the exploits of Captain Nemo, the series follows young inventor Jean and former circus performer Nadia, who are led off to adventure by a secret in Nadia's pendant.

The original Japanese broadcast of the series aired from April 1990 to April 1991 for 39 episodes. Streamline Pictures released eight episodes in the United States on VHS from March 1992 to August 1993. The full series was distributed in the United States from 2001 to 2002 by ADV Films on DVD and VHS, with an American broadcast airing on ADV's Anime Network. Following the 2009 closure of ADV, Sentai Filmworks re-licensed the series and re-released it to Blu-ray and DVD in March 2014. GKIDS acquired the license in 2022 and released a new 4K restoration the same year.