The vadose zone (from Latin for "shallow" and also known as the unsaturated zone) is part of the aquifer that lies between the land surface and the top of the phreatic zone. The upper boundary of the vadose zone is thus the ground level and the lower boundary is the water table, which is the position beneath which groundwater fills all the pores in rock.

Water in the vadose zone has pressure head less than atmospheric pressure and is retained by a combination of adhesion, and capillary action. If the vadose zone envelops soil, the water contained therein is termed soil moisture.

Understanding the movement of water within the vadose zone is extremely important for evaluation of aquifer recharge, water quality and contaminant transport, and stormwater removal and flood control.