Scripture does not name her directly, but Genesis records that Adam and Eve had many other sons and daughters, so Cain most likely married a close relative — a sister or niece — which was not yet prohibited at that early point in human history.
Genesis 5:4 states that Adam lived many years after Seth's birth and had additional sons and daughters, indicating a growing human family beyond the individuals named in the early chapters of Genesis. Since Scripture teaches that the entire human race descended from a single first pair, Cain's wife would necessarily have been a close relative, most likely a sister or niece. This kind of marriage was not forbidden until much later, when the Mosaic law restricted close-relative marriage, likely both for moral order and because accumulated genetic effects had made such unions medically riskier by that time. Scripture is intentionally brief here, focused on the moral and spiritual lessons of Cain's story rather than exhaustive genealogical detail, so some uncertainty about the exact details remains appropriate.
These themes are best read through the whole testimony of Scripture, emphasizing God's self-revelation in Christ, the active ministry of the Holy Spirit, and the restoration of the whole person. Bible study should lead to a changed life, compassionate service, and a clearer picture of God's loving character.