The mark of Cain is one of those biblical curiosities that people sometimes ask about.
In Genesis 4, when Cain is worried that people will find him and kill him to avenge his murder of Abel, God replies: “Not so! If anyone kills Cain, vengeance shall be taken on him sevenfold.” And Yahweh put a mark on Cain, lest any who found him should attack him” (Genesis 4:15, ESV).
This “mark” is often seen as a mystery, like Paul’s “thorn in the flesh.” While it is probably not that important that we know exactly what it was, people do wonder.
In his commentary on Genesis, Gordon Wenham says, “The nature of Cain’s sign or mark has been the subject of endless inconclusive speculation.” Since the sign is meant to deter people who might attack Cain, the majority of writers have concluded “that the mark of Cain must be something about him that shows he has divine protection, e.g., a tattoo, special hairstyle, or the like.” Wenham admits that there is little evidence for any of the suggestions, but thinks the “simplest” is the idea that Cain’s name is itself the sign, since it “sounds somewhat like” the Hebrew verb “shall be punished.” That seems like a reach. An explanation of Cain’s name was given at the beginning of chapter 4 (it is from the verb “acquire” or “gain”), and there is no hint in 4:15 that the sign has any connection with it. The mark/sign appears to be something that comes into play at this particular point in time.
In another commentary, Bruce Waltke asserts (with no reason offered) that “This is apparently a protective tattoo.” John Calvin is characteristically more restrained and simply says, “It may suffice for us, that there was some visible token which should repress in the spectators the desire and the audacity to inflict injury.”
One suggestion I have not seen, but which seems to me fairly obvious, is that the “mark” was astral—a sign in the heavens. In this verse, I think the translation of the New American Standard Bible is more literal, and better, than the English Standard Version.
In the NASB, Genesis 4:15 says: “And the Lord appointed a sign for Cain, so that no one finding him would slay him.”
Two linguistic notes on the difference here. First, what the ESV translates as “mark” the NASB translates as “sign,” which is a better rendering of the Hebrew (א֔וֹת). “Mark” gives the impression of something about Cain’s person (hence suggestions of tattoos or the like). “Sign” is more generic and has a broader range of possible references. Second, this is not a mark or sign “on” Cain (per ESV), but a sign “for” Cain. There is a preposition in Hebrew that would mean “on” (בְּ), but this is not it. Used instead is a preposition meaning generically something more like “with regard to” (לְ).
Now what could the “sign for (or regarding) Cain” be? We already have a precedent for “signs” in Genesis. In the creation story, it is what the sun, moon, and stars were made for. To give light, rule day and night, mark time, and to be for “signs” (א֔וֹת) and seasons (Genesis 1:14ff).
This seems to me the most straightforward and plausible explanation. In the end, we are not told exactly what the sign is. It is not the point of the story. But a sign in the heavens makes sense here and would represent the first of many in the biblical narrative.