地下水（英语：Groundwater）顾名思义，就是地面以下的水，是贮存于地面以下岩石裂缝和土壤空隙中的水，按形态分为气态水、吸着水、薄膜水、毛细管水、重力水、固态水等。

地下水一般是硬水，现在可行抽水深度以上水量约为4,200,000立方千米，通过水循环与其他水体交换，在地表下亦缓慢移动。地下水水量稳定，很少受气候影响，污染程度低，可作为居民生活用水、工业用水以及农业灌溉水源。此外，地下水也是生态环境的重要因素和一种活跃的地质营力与信息载体。

Groundwater (or ground water) is the water present beneath Earth's surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from, and eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology.