Knife in the Water (Polish: Nóż w wodzie) is a 1962 Polish psychological thriller film co-written and directed by Roman Polanski in his feature debut, and starring Leon Niemczyk, Jolanta Umecka, and Zygmunt Malanowicz. Its plot follows a husband and wife who are accompanied on a boating trip by a young male hitchhiker, who spurs a number of escalating confrontations between the couple.

The film was nominated for Academy Award for Best Foreign Language Film and is Polanski's only Polish-language feature to date. Knife in the Water has garnered acclaim from film critics since its release, and is one of Polanski's best-reviewed works. American filmmaker Martin Scorsese recognized the film as one of the masterpieces of Polish cinema and in 2013 he selected it for screening alongside films such as Andrzej Wajda's Ashes and Diamonds and Innocent Sorcerers in the United States, Canada and United Kingdom as part of the Martin Scorsese Presents: Masterpieces of Polish Cinema festival of Polish films.