The water organ or hydraulic organ (Greek: ὕδραυλις) (early types are sometimes called 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.

The hydraulic organ is often confused with the hydraulis. The hydraulis is the name of a Greek instrument created by Ctesibius of Alexandria. The hydraulis has a reservoir of air which is inserted into a cistern of water. The air is pushed into the reservoir with hand pumps, and exits the reservoir as pressurized air to blow through the pipes. The reservoir is open on the bottom, allowing water to maintain the pressure on the air as the air supply fluctuates from either the pumps pushing more air in, or the pipes letting air out.

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.