Who was Cain’s wife?

The Bible does not say the name of Cain’s wife. The most reasonable conclusion, according to the narrative itself, is that she was a sister or niece (a close descendant) of Cain.

Genesis shows that Adam and Eve had more sons and daughters besides Cain and Abel (Genesis 5:4). In that early stage of humanity, their descendants had no one to marry outside their own family.

Later, when the population increased, God gave laws that prohibited close relationships within the family (Leviticus 18:6–18). That indicates the prohibition appears later, when it was already possible to avoid it.