The Book of Genesis (from Greek Γένεσις, Génesis; Hebrew: בְּרֵאשִׁית Bəreʾšīt, "In [the] beginning") is the first book of the Hebrew Bible and the Christian Old Testament. Its Hebrew name is the same as its first word, Bereshit ("In the beginning"). Genesis is an account of the creation of the world, the early history of humanity, and of Israel's ancestors and the origins of the Jewish people.

Tradition credits Moses as the author of Genesis, as well as the books of Exodus, Leviticus, Numbers and most of Deuteronomy; however, modern scholars, especially from the 19th century onward, place the books' authorship in the 6th and 5th centuries BC, hundreds of years after Moses is supposed to have lived. Based on scientific interpretation of archaeological, genetic, and linguistic evidence, most mainstream Bible scholars consider Genesis to be primarily mythological rather than historical.

It is divisible into two parts, the primeval history (chapters 1–11) and the ancestral history (chapters 12–50). The primeval history sets out the author's concepts of the nature of the deity and of humankind's relationship with its maker: God creates a world which is good and fit for mankind, but when man corrupts it with sin God decides to destroy his creation, sparing only the righteous Noah and his family to re-establish the relationship between man and God. The ancestral history (chapters 12–50) tells of the prehistory of Israel, God's chosen people. At God's command, Noah's descendant Abraham journeys from his birthplace (described as Ur of the Chaldeans and whose identification with Sumerian Ur is tentative in modern scholarship) into the God-given land of Canaan, where he dwells as a sojourner, as does his son Isaac and his grandson Jacob. Jacob's name is changed to "Israel", and through the agency of his son Joseph, the children of Israel descend into Egypt, 70 people in all with their households, and God promises them a future of greatness. Genesis ends with Israel in Egypt, ready for the coming of Moses and the Exodus (departure). The narrative is punctuated by a series of covenants with God, successively narrowing in scope from all mankind (the covenant with Noah) to a special relationship with one people alone (Abraham and his descendants through Isaac and Jacob).

In Judaism, the theological importance of Genesis centres on the covenants linking God to his chosen people and the people to the Promised Land.