The only mention of Cain’s wife is in Genesis 4:17, where Cain and his wife had a son. They soon named a city after their son. The Bible does say that Adam and Eve had other children besides Cain and Abel (Genesis 5:4). No doubt it was one of these daughters that Cain took for a wife.
God commanded humanity to be fruitful and multiply (Genesis 1:28; 9:1) and thus it was necessary to marry one’s own relative. Abraham, Isaac and Jacob married their own relatives so as to avoid intermarrying non-believers. Later however, Moses commanded close intermarriages between members of the same family to be prohibited (see Leviticus 18:6-17).
Only one verse in the Bible, Genesis 4:17, mentions Cain’s wife, however most of Genesis chapter 4 tells the story of Cain. The high-points of this chapter include: the birth of Cain and Abel, the sacrifice both brothers bring to the Lord, Cain murdering his brother and Cain & his wife’s son. The chapter ends with Adam and Eve having another son named Seth. This is what Genesis 4:25 says, "And Adam knew his wife again, and she bore a son and named him Seth, 'For God has appointed another seed for me instead of Abel, whom Cain killed.'”