The antediluvian period was the period that preceded the Great Flood of Noah as related in the Book of Genesis in the Hebrew Bible. It was an important feature in versions of natural history that were formerly much more widely accepted, but accepted by creationists. The primary source of knowledge about the antediluvian period is Genesis, chapters 5 and 6.

The antediluvian period

Creationists such as William Whiston (A New Theory of the Earth 1696) and Henry Morris (The Genesis Flood 1961) typically describe the antediluvian period as follows:

• People lived much longer than people today, typically between 700-950 years, as reported in the Genealogies of Genesis;
• The Earth contained many more people than the Earth than today. Whiston calculated that as many as 500 million humans may have been born in the antediluvian period, based on assumptions about lifespans and fertility rates;
• There were no clouds or rain. Instead, the Earth was watered by mists which rose from the Earth.

The Bible speaks of this era as being a time of great wickedness. There were Gibborim (giants) in the earth in those days as well as Nephilim, some translations identify the two as one and the same. These giants were the offspring of the "sons of God", usually interpreted as angels, who descended from the heavens to beget them with mortal women. The Gibborim were unusually powerful; Genesis calls them "heroes of old, men of renown;" (Enoshi Ha Shem). Genesis also says at that time mankind was an evil race, whom God repented of creating, and therefore he sent the Flood to wipe out all human life, all except Noah and his family. Nevertheless, the Nephilim (literally meaning fallen ones) reappear much later in the Biblical narrative, in Numbers 13:31-33.

Flood geology holds that fossils, seashells on mountaintops, fossil fuels, submarine canyon extensions, and many other geological formations are best explained by a global flood.

"Antediluvian" is sometimes used figuratively to refer to anything of great age.

In Vampire:_The_Masquerade the Antediluvians are the most ancient of vampires, who have remained in hiding or suspended animation from the time of the flood of Noah. Their return is a sign of the end times.