A smear campaign, also referred to as a smear tactic or simply a smear, is an effort to damage or call into question someone's reputation, by propounding negative propaganda. It makes use of discrediting tactics. It can be applied to individuals or groups. Common targets are public officials, politicians, heads of state, political candidates, activists, celebrities (especially those who are involved in politics), and ex-spouses. The term also applies in other contexts, such as the workplace. The term smear campaign became popular around the year 1936.

A smear campaign is an intentional, premeditated effort to undermine an individual's or group's reputation, credibility, and character. Like negative campaigning, most often smear campaigns target government officials, politicians, political candidates, and other public figures. However, public relations campaigns might also employ smear tactics in the course of managing an individual or institutional brand to target competitors and potential threats.

Smear campaigns can also be used as a campaign tactic associated with tabloid journalism, which is a type of journalism that presents little well-researched news and instead uses eye-catching headlines, scandal-mongering and sensationalism. For example, during Gary Hart's 1988 presidential campaign (see below), the New York Post reported on its front page big, black block letters: "GARY: I'M NO WOMANIZER."

Deflection has been described as a wrap-up smear: "You make up something. Then you have the press write about it. And then you say, everybody is writing about this charge".

Smear tactics are commonly used to undermine effective arguments or critiques.

During the 1856 presidential election, John C. Frémont was the target of a smear campaign alleging that he was a Catholic, among other accusations. The campaign was designed to undermine support for Fremont from those who were suspicious of Catholics.

Ralph Nader was the victim of a smear campaign during the 1960s, when he was campaigning for car safety. In order to smear Nader and deflect public attention from his campaign, General Motors engaged private investigators to search for damaging or embarrassing incidents from his past. In early March 1966, several media outlets, including The New Republic and The New York Times, reported that GM had tried to discredit Nader, hiring private detectives to tap his phones and investigate his past and hiring prostitutes to trap him in compromising situations. Nader sued the company for invasion of privacy and settled the case for $284,000. Nader's lawsuit against GM was ultimately decided by the New York Court of Appeals, whose opinion in the case expanded tort law to cover "overzealous surveillance." Nader used the proceeds from the lawsuit to start the pro-consumer Center for Study of Responsive Law.

Gary Hart was the target of a smear campaign during the 1988 US presidential campaign. The New York Post once reported on its front page big, black block letters: "GARY: I'M NO WOMANIZER."