The Ogallala Aquifer is a shallow water table aquifer surrounded by sand, silt, clay, and gravel located beneath the Great Plains in the United States. As one of the world's largest aquifers, it underlies an area of approximately 174,000 sq mi (450,000 km2) in portions of eight states. It was named in 1898 by geologist N. H. Darton from its type locality near the town of Ogallala, Nebraska. The aquifer is part of the High Plains Aquifer System, and resides in the Ogallala Formation, which is the principal geologic unit underlying 80% of the High Plains.

An aquifer is an underground layer of water-bearing material, consisting of permeable or fractured rock, or of unconsolidated materials. Aquifers vary greatly in their characteristics. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Related terms include aquitard, which is a bed of low permeability along an aquifer, and aquiclude, which is a solid, impermeable area underlying or overlying an aquifer, the pressure of which could lead to the formation of a confined aquifer. The classification of aquifers is as follows: Saturated versus unsaturated; aquifers versus aquitards; confined versus unconfined; isotropic versus anisotropic; porous, karst, or fractured; transboundary aquifer.