水刀（英语：Water jet cutter）是将水流以高压方式喷出的水柱去切割物体的一种工具。水刀特点在于没有热应力，因此可在无高温的切割过程中，被切割物没有化学变化，也不会变形。

水刀依输出的压力分为超高压和低压两种型式，超高压型在工作上需求压力为250-600Mpa，对金属工业的机械加工较常为应用；另一种低压型，其工作上需求压力为25~50Mp，这类的水刀应用于不能有火光、高温的场合，所以也被称为「安全水刀」。

A water jet cutter, also known as a water jet or waterjet, is an industrial tool capable of cutting a wide variety of materials using a very high-pressure jet of water, or a mixture of water and an abrasive substance. The term abrasivejet refers specifically to the use of a mixture of water and abrasive to cut hard materials such as metal or granite, while the terms pure waterjet and water-only cutting refer to waterjet cutting without the use of added abrasives, often used for softer materials such as wood or rubber.