'For water wheels used to drive boats, see paddle wheel. For wheels used solely to lift water see noria. For factories or industries driven by water wheels see watermill.

A water wheel is a means of extracting power from the flow (or fall) of water, that is hydropower. Water wheels were widely used in the Middle Ages to power industry in Europe. The alternatives were the windmill and human and animal power. The most common use of the water wheel was to mill flour in gristmills, but other uses included foundry work and machining, and pounding linen for use in paper.

A water wheel consists of a large wooden or metal wheel, with a number of blades or buckets arranged on the outside rim forming the driving surface. Most commonly, the wheel is mounted vertically on a horizontal axle, but the tub or Norse wheel is mounted horizontally on a vertical shaft. Vertical wheels can transmit power either through the axle or via a ring gear and typically drive belts or gears; horizontal wheels usually directly drive their load.

A flowing stream was often dammed in order to maintain a steady supply of water for the mill; the dammed water would form a mill pond. A channel created for the water to follow while flowing to or from a water wheel is a mill race (also spelled millrace) or simply a "race", and is customarily divided into sections. The race bringing water from the mill pond to the water wheel is a headrace; the one carrying water after it has left the wheel is commonly referred to as a tailrace.