A hyperthermophile is an organism that thrives in extremely hot environments—from 60 °C (140 °F) upward. An optimal temperature for the existence of hyperthermophiles is often above 80 °C (176 °F). Hyperthermophiles are often within the domain Archaea, although some bacteria are also able to tolerate extreme temperatures. Some of these bacteria are able to live at temperatures greater than 100 °C, deep in the ocean where high pressures increase the boiling point of water. Many hyperthermophiles are also able to withstand other environmental extremes, such as high acidity or high radiation levels. Hyperthermophiles are a subset of extremophiles. Their existence may support the possibility of extraterrestrial life, showing that life can thrive in environmental extremes.

Hyperthermophiles isolated from hot springs in Yellowstone National Park were first reported by Thomas D. Brock in 1965. Since then, more than 70 species have been established. The most extreme hyperthermophiles live on the superheated walls of deep-sea hydrothermal vents, requiring temperatures of at least 90 °C for survival. An extraordinary heat-tolerant hyperthermophile is Geogemma barossii (Strain 121), which has been able to double its population during 24 hours in an autoclave at 121 °C (hence its name). The current record growth temperature is 122 °C, for Methanopyrus kandleri.

Although no hyperthermophile has shown to thrive at temperatures >122 °C, their existence is possible. Strain 121 survives 130 °C for two hours, but was not able to reproduce until it had been transferred into a fresh growth medium, at a relatively cooler 103 °C.

Early research into hyperthermophiles speculated that their genome could be characterized by high guanine-cytosine content; however, recent studies show that "there is no obvious correlation between the GC content of the genome and the optimal environmental growth temperature of the organism."

The protein molecules in the hyperthermophiles exhibit hyperthermostability—that is, they can maintain structural stability (and therefore function) at high temperatures. Such proteins are homologous to their functional analogs in organisms that thrive at lower temperatures but have evolved to exhibit optimal function at much greater temperatures. Most of the low-temperature homologs of the hyperthermostable proteins would be denatured above 60 °C. Such hyperthermostable proteins are often commercially important, as chemical reactions proceed faster at high temperatures.

Due to their extreme environments, hyperthermophiles can be adapted to several variety of factors such as pH, redox potential, level of salinity, and temperature. They grow (similar to mesophiles) within a temperature range of about 25–30 °C between the minimal and maximal temperature. The fastest growth is obtained at their optimal growth temperature which may be up to 106 °C. The main characteristics they present in their morphology are:

Hyperthermophiles have a great diversity in metabolism including chemolithoautotrophy and chemoorganoheterotrophy, while there are no phototrophic hyperthermophiles known. Sugar catabolism involves non-phosphorylated versions of the Entner-Doudoroff pathway some modified versions of the Embden-Meyerhof pathway, the canonical Embden-Meyerhof pathway being present only in hyperthermophilic bacteria but not archaea.

Most of what is known about sugar catabolism in hyperthermophiles comes from observation on Pyrococcus furiosus. It grows on many different sugars such as starch, maltose, and cellobiose, that once in the cell are transformed to glucose, but other organic substrates can be used as carbon and energy sources.