Who is Cain's wife?

Short Answer: His sister or possibly his niece.

The name and origin of Cain's wife has long been a mystery for many people. No woman, other than Eve, is mentioned in the Bible before her. She seems to appear from nowhere. In fact, Cain's wife is only mentioned very briefly as bearing a son named Enoch, whom we will discuss at some other time.

I believe we can identify her as follows:
According to Genesis, Adam was the first man and Eve was the first woman. While Cain and Abel are the first mentioned children of Adam and Eve, it is very unlikely that they were the only children born of this pair before the time of the first murder. After all, God specifically told them while in the Garden of Eden to be "fruitful and multiply". It is a fact that a woman can potentially have a child approximately once a year throughout her fertile life. The actual ages of Cain and Abel are unknown, however, it seems apparent that both are adults, since they are both successful in their chosen trades. Genesis 4:3 suggests that a significant amount of time has passed since Cain and Abel's birth by mentioning their offerings from their professions. With this being said, Cain and Abel likely had several brothers and sisters, who may have also begun having children of their own. There is a simple reason for their siblings not being mentioned in the Bible: To mention every child from Adam and Eve would lengthen Genesis extensively and would likely bore the reader. This is common sense. Any author will not bother mentioning characters that are not important to the story.

By the time these brothers were able to become successful in their trades, there would have been a significant number of women available as wives. Although the wife of Cain isn't clearly identified, by name or any other information, these speculations, facts, and lines of logic strongly suggest that she is likely either a sister or niece.