A red herring is something that misleads or distracts from a relevant or important question. It may be either a logical fallacy or a literary device that leads readers or audiences toward a false conclusion. A red herring may be used intentionally, as in mystery fiction or as part of rhetorical strategies (e.g., in politics), or may be used in argumentation inadvertently.

The term was popularized in 1807 by English polemicist William Cobbett, who told a story of having used a strong-smelling smoked fish to divert and distract hounds from chasing a rabbit.

As an informal fallacy, the red herring falls into a broad class of relevance fallacies. Unlike the straw man, which involves a distortion of the other party's position, the red herring is a seemingly plausible, though ultimately irrelevant, diversionary tactic. According to the Oxford English Dictionary, a red herring may be intentional or unintentional; it is not necessarily a conscious intent to mislead.

The expression is mainly used to assert that an argument is not relevant to the issue being discussed. For example, "I think we should make the academic requirements stricter for students. I recommend you support this because we are in a budget crisis, and we do not want our salaries affected." The second sentence, though used to support the first sentence, does not address that topic.

In fiction and non-fiction, a red herring may be intentionally used by the writer to plant a false clue that leads readers or audiences toward a false conclusion. For example, the character of Bishop Aringarosa in Dan Brown's The Da Vinci Code is presented for most of the novel as if he is at the centre of the church's conspiracies, but is later revealed to have been innocently duped by the true antagonist of the story. The character's name is a loose Italian translation of "red herring" (aringa rosa; rosa actually meaning 'pink', and very close to rossa, 'red').

A red herring is found in the first Sherlock Holmes story, A Study in Scarlet, where the murderer writes at the crime scene the word Rache ('revenge' in German), leading the police—and the reader—to mistakenly presume that a German was involved.

A red herring is often used in legal studies and exam problems to mislead and distract students from reaching a correct conclusion about a legal issue, intended as a device that tests students' comprehension of underlying law and their ability to properly discern material factual circumstances.

When I was a boy, we used [to], in order to draw off the harriers from the trail of a hare that we had set down as our own private property, get to her haunt early in the morning, and drag a red-herring, tied to a string, four or five miles over hedges and ditches, across fields and through coppices, till we got to a point, whence we were pretty sure the hunters would not return to the spot where they had [been] thrown off; and, though I would, by no means, be understood, as comparing the editors and proprietors of the London daily press to animals half so sagacious and so faithful as hounds, I cannot help thinking, that, in the case to which we are referring, they must have been misled, at first, by some political deceiver.