A piezophile (from Greek "piezo-" for pressure and "-phile" for loving) is an organism with optimal growth under high hydrostatic pressure i.e. an organism that has its maximum rate of growth at a hydrostatic pressure equal to or above 10 MPa (= 99 atm = 1,450 psi), when tested over all permissible temperatures. Originally, the term barophile was used for these organisms, but since the prefix "baro-" stands for weight, the term piezophile was given preference. Like all definitions of extremophiles, the definition of piezophiles is anthropocentric, and humans consider that moderate values for hydrostatic pressure are those around 1 atm (= 0.1 MPa = 14.7 psi). Hyperpiezophiles are organisms that have their maximum growth rate above 50 MPa (= 493 atm = 7,252 psi).

Though the high hydrostatic pressure has deleterious effects on organisms growing at atmospheric pressure, these organisms which are solely found at high pressure habitats at deep sea in fact need high pressures for their optimum growth. Often their growth is inhibited at much higher pressures (such as 100MPa) compared to those organisms which normally grow at low pressures.

The first obligate piezophile found was a psychrophilic bacteria called Colwellia marinimaniae strain M-41. It was isolated from a decaying amphipod Hirondellea gigas from the bottom of Mariana Trench. The first thermophilic piezophilic archaea Pyrococcus yayanosii strain CH1 was isolated from the Ashadze site, a deep sea hydrothermal vent. Strain MT-41 has a optimal growth pressure at 70MPa at 2 °C and strain CH1 has a optimal growth pressure at 52MPa at 98 °C. They are unable to grow at pressures lower than or equal to 20MPa, and both can grow at pressures above 100MPa.The current record for highest hydrostatic pressure where growth was observed is 140MPa shown by Colwellia marinimaniae MTCD1. The term "obligate piezophile" refers to organisms that are unable to grow under lower hydrostatic pressures, such as 0.1 MPa. In contrast, piezotolerant organisms are those that have their maximum rate of growth at a hydrostatic pressure under 10 MPa, but that nevertheless are able to grow at lower rates under higher hydrostatic pressures.

Most of the Earth's biosphere (in terms of volume) is subject to high hydrostatic pressure, and the piezosphere comprises the deep sea (at the depth of 1,000 m and greater) plus the deep subsurface (which can extend up to 5,000 m beneath the seafloor or the continental surface). The deep sea has a mean temperature around 1 to 3 °C, and it is dominated by psychropiezophiles. In contrast, deep subsurface and hydrothermal vents in the seafloor are dominated by thermopiezophiles that prosper in temperatures above 45 °C (113 °F).

Although the study of nutrient acquisition and metabolism within the piezosphere is still in its infancy, it is understood that most of the organic matter present are refractory complex polymers from the eutrophic zone. Both heterotrophic metabolism and autotrophic fixation are present within the piezosphere and additional research suggests significant metabolism of iron-bearing minerals and carbon monoxide. Additional research is required to fully understand and characterize piezosphere metabolism.