Quack or Quacks may refer to:
- A person who practices quackery (the promotion of unproven or fraudulent medical practices)
- Hendrick Peter Godfried Quack, Dutch economist and historian
- Quack (horse) (1969–1995), an American Thoroughbred racehorse
- The Quack (horse), a 19th-century Australian Thoroughbred racehorse
- Quack (sound), onomatopoeia for the sound made by ducks
- Quack (album), an album by the American-Canadian group Duck Sauce
- Quacks (TV series), a British comedy series about quackery
- Quack grass, a type of grass
- Quack.com, a company based in Silicon Valley
- O. K. Quack, a character in the Scrooge McDuck universe
- Quack, a duck in Peep and the Big Wide World, a children's TV series
- Quack, a portmanteau of queen and jack; see Glossary of contract bridge terms#Quack