中水（Reclaimed water），也叫再生水、再造水或回收水，是经过处理的污水回收再用。因为城市建设中将供水称为“上水”，污水排放称为“下水”，所以中水取其两者之间的意思。

中水有城市污水处理厂集中处理回用的，也有一个社区甚至一座单独住房自行回用的。社区或住房小范围的中水，一般只收集比较清洁的污水，如洗澡水、游泳池水、厨房排水等进行简单的处理，如过滤、沉淀等。城市集中回用的中水需单独设置管网，因此投资较大，但可以节约水资源，并减少污水排放量，因此可以大大地减少供水和污水处理费用。

中水回收用主要用于马桶、尿兜冲厕、绿地灌溉、冲洗街道、洗车、工业用清洗（如电子业）、空调系统冷却等。

Reclaimed water or recycled water, is former wastewater (sewage) that is treated to remove solids and impurities, and used in sustainable landscaping irrigation, to recharge groundwater aquifers, to meet commercial and industrial water needs, and for drinking. The purpose of these processes is water conservation and sustainability, rather than discharging the treated water to surface waters such as rivers and oceans. In some cases, recycled water can be used for streamflow augmentation to benefit ecosystems and improve aesthetics. One example of this is along Calera Creek in the City of Pacifica, CA.