A paleoatmosphere (or palaeoatmosphere) is an atmosphere, particularly that of Earth, at some unspecified time in the geological past.
When regarding geological history of Earth, the paleoatmosphere can be chronologically divided into the Hadean first atmosphere, which resembled the compositions of the solar nebula; the Archean second atmosphere (also known as the prebiotic atmosphere), which became nitrogen-abundant due to volcanic outgassing and meteoric injections during the Late Heavy Bombardment; and the Proterozoic and Phanerozoic third atmosphere, which started to contain free oxygen due to biotic photosynthesis.