The Secchi disk (or Secchi disc), as created in 1865 by Angelo Secchi, is a plain white, circular disk 30 cm (12 in) in diameter used to measure water transparency or turbidity in bodies of water. The disc is mounted on a pole or line, and lowered slowly down in the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the Secchi depth and is related to water turbidity. Since its invention, the disk has also been used in a modified, smaller 20 cm (8 in) diameter, black and white design to measure freshwater transparency.

Similar disks, with a black-and-yellow pattern, are used as fiducial markers on vehicles in crash tests, crash-test dummies, and other kinetic experiments.

Angelo Secchi (Italian pronunciation: [ˈandʒelo ˈsekki]; 28 June 1818 – 26 February 1878) was an Italian Catholic priest, astronomer from the Italian region of Emilia. He was director of the observatory at the Pontifical Gregorian University (then called the Roman College) for 28 years. He was a pioneer in astronomical spectroscopy, and was one of the first scientists to state authoritatively that the Sun is a star.