A sluice is a water channel that is controlled at its head by a gate (from the Dutch word 'sluis'). For example, a millrace is a sluice that channels water toward a water mill. The terms "sluice gate", "knife gate", and "slide gate" are used interchangeably in the water/wastewater control industry.

A sluice gate is traditionally a wooden or metal plate that slides in grooves in the sides of the channel. Sluice gates are commonly used to control water levels and flow rates in rivers and canals. They are also used in wastewater treatment plants and to recover minerals in mining operations, and in watermills.

The term sluice gate refers to any gate that operates by allowing water to flow under it. When a sluice gate is fully lowered, water sometimes spills over the top, in which case the gate operates as a weir.

Usually, a mechanism drives the sluice gate up or down. This may be a simple, hand-operated, worm drive or rack-and-pinion drive, or it may be electrically or hydraulically powered.

The gates of a Guillotine lock work in a way similar to a sluice gate, but most canal lock gates are hinged to swing like doors.