The bacteria-like organism lives in an inhospitable undersea environment in toxic boiling water at pressures that would instantly crush most things to death.
There is no light either due to the thick black smoke that pores out of vents called black smokers that surround its home.
The discovery suggests that life could exist on planets very different from Earth. It also suggests that life did not always evolve in the ways biology teaches - in warm, soupy waters bathed in sunlight on the planet's surface.
Kazim al-Kashafi and Derek Lovley of the University of Massachusetts tested a sample of water collected about 320km off Puget Sound and 2km below the surface of the Pacific Ocean.
The water was collected by a University of Washington team looking for archaea, bacteria-like organisms that live in extreme environments.
The area they explored can only be reached by remotely operated submarines.
Known as the Juan de Fuca Ridge, it is marked by black smokers that rise the equivalent height of a four-storey building.
Other tiny microbes, teetering in the balance between frigid and boiling waters, often use sulphur in the water as fuel and are known to survive temperatures of up to 113C (235 Fahrenheit).
But the newly discovered microbe survives even higher temperatures and does not use either oxygen or sulphur in respiration.
Instead it uses iron to burn its food for energy - the role played by oxygen on most other species on Earth.
"It's a novel form of respiration," Lovley said in a statement.
Al-Kashafi and Lovley tested their sample by steaming it in an autoclave - used to disinfect medical equipment.
To their surprise, they were able to grow this organism even after bringing the water to temperatures far above the 100C boiling point of water – in fact around 121C (250 degrees Fahrenheit).
They nicknamed it Strain 121.
"Growth at 121C is remarkable because sterilization at 121C … is a standard procedure shown to kill all previously described microorganisms and heat-resistant spores," they wrote in a report published in the journal Science.
"Autoclaving did not kill strain 121, and it doubled in cell numbers after 24 hours at 121C."