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China: Clepsydra or water clock from Mancheng Tomb 1, 2nd century BCE, Hebei Provincial Museum, Shijiazhuang

A water clock or clepsydra (Greek) is any timepiece in which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel where the amount is then measured.

Water clocks, along with sundials, are likely to be the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick. Where and when they were first invented is not known, and given their great antiquity it may never be. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BCE.

Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, claim that water clocks appeared in China as early as 4000 BCE.