The Pre-Noachian period is a geological system and early time period on Mars marked by intense meteoroid and asteroid impacts, volcanic and tectonic activity, and the potential presence of surface or subsurface water. The era holds significant importance in Mars' history as it witnessed the planet's formation and the shaping of its geological features. However, understanding the Pre-Noachian period remains elusive, as it is the least comprehended among Mars' four geological epochs. Much of the evidence from this period has been obscured by erosion and deposition processes.

During the Pre-Noachian period, the northern hemisphere of Mars developed lowlands, which may have served as reservoirs for ancient Martian water during subsequent epochs. These lowlands likely formed through lava erosion caused by extensive volcanic activity, as the Pre-Noachian period experienced the highest level of volcanic activity among all Martian epochs.

The atmosphere of Mars during the Pre-Noachian period was denser than it is today, containing higher concentrations of carbon dioxide from volcanic and meteorite outgassing. This may have led to a greenhouse effect on Mars and the formation of various minerals, including silicates, iron oxide, sulfates, carbonates, clays, and hydrates, due to intense heat. Meteorite outgassing is believed to have caused water vapor to condense into Mars' atmosphere, eventually precipitating onto the surface and forming oceans during subsequent epochs. Over time, gases trapped by Mars' gravity during the Pre-Noachian period have gradually escaped into space as the planet cooled.

During this era, Mars possessed a distinctive magnetic field, indicating an active core. As magma cooled in the planet's lower layers, it generated metals necessary for the formation of the magnetic field.

The Pre-Noachian System and Period is named after Noachis Terra, a region of highlands west of the Hellas Planitia basin, and the pre comes from the word meaning before.

Martian time periods are based on geological mapping of surface units from spacecraft images. A surface unit is a terrain with a distinct texture, color, albedo, spectral property, or set of landforms that distinguish it from other surface units and is large enough to be shown on a map. Mappers use a stratigraphic approach pioneered in the early 1960s for photogeologic studies of the Moon. Although based on surface characteristics, a surface unit is not the surface itself or group of landforms. It is an inferred geologic unit (e.g., formation) representing a sheetlike, wedgelike, or tabular body of rock that underlies the surface. A surface unit may be a crater ejecta deposit, lava flow, or any surface that can be represented in three dimensions as a discrete stratum bound above or below by adjacent units (illustrated right). Using principles such as superpositioning (illustrated left), cross-cutting relationships, and the relationship of impact crater density to age, geologists can place the units into a relative age sequence from oldest to youngest. Units of similar age are grouped globally into larger, time-stratigraphic (chronostratigraphic) units, called systems. For Mars, four systems are defined: the Pre-Noachian Noachian, Hesperian, and Amazonian. Geologic units lying below (older than) the Noachian are informally designated Pre-Noachian. The geologic time (geochronologic) equivalent of the Pre-Noachian System is the Pre-Noachian Period. Rock or surface units of the Pre-Noachian System were formed or deposited during the Pre-Noachian Period.

System and Period are not interchangeable terms in formal stratigraphic nomenclature, although they are frequently confused in popular literature. A system is an idealized stratigraphic column based on the physical rock record of a type area (type section) correlated with rocks sections from many different locations planetwide. A system is bound above and below by strata with distinctly different characteristics (on Earth, usually index fossils) that indicate dramatic (often abrupt) changes in the dominant fauna or environmental conditions. (See Cretaceous–Paleogene boundary as example.)

At any location, rock sections in a given system are apt to contain gaps (unconformities) analogous to missing pages from a book. In some places, rocks from the system are absent entirely due to nondeposition or later erosion. For example, rocks of the Cretaceous System are absent throughout much of the eastern central interior of the United States. However, the time interval of the Cretaceous (Cretaceous Period) still occurred there. Thus, a geologic period represents the time interval over which the strata of a system were deposited, including any unknown amounts of time present in gaps. Periods are measured in years, determined by radioactive dating. On Mars, radiometric ages are not available except from Martian meteorites whose provenance and stratigraphic context are unknown. Instead, absolute ages on Mars are determined by impact crater density, which is heavily dependent upon models of crater formation over time. Accordingly, the beginning and end dates for Martian periods are uncertain, especially for the Hesperian/Amazonian boundary, which may be in error by a factor of 2 or 3.