The water organ or hydraulic organ (ὕδραυλις) (early types are sometimes called hydraulis, hydraulos, hydraulus or hydraula) is a type of pipe organ blown by air, where the power source pushing the air is derived by water from a natural source (e.g. by a waterfall) or by a manual pump. Consequently, the water organ lacks a bellows, blower, or compressor.

On the water organ, since the 15th century, the water is also used as a source of power to drive a mechanism similar to that of the Barrel organ, which has a pinned barrel that contains a specific song to be played. The hydraulis in ancient Greek is often imagined as an automatic organ, but there is no source evidence for it. One of the oldest automatic instruments known is the automatic flute player invented by Banū Mūsā brothers in 9th-century Arabia.