"Just Say No" was an advertising campaign prevalent during the 1980s and early 1990s as a part of the U.S.-led war on drugs, aiming to discourage children from engaging in illegal recreational drug use by offering various ways of saying no. The slogan was created and championed by Nancy Reagan during her husband's presidency.

The campaign emerged from a substance abuse prevention program supported by the National Institutes of Health, pioneered in the 1970s by University of Houston Social Psychology Professor Richard I. Evans. Evans promoted a social inoculation model, which included teaching student skills to resist peer pressure and other social influences. The campaign involved University projects done by students across the nation. Jordan Zimmerman, then a student at University of South Florida, and later an advertising entrepreneur, won the campaign. The anti-drug movement was among the resistance skills recommended in response to low peer pressure, and Nancy Reagan's larger campaign proved to be an effective dissemination of this social inoculation strategy.

Nancy Reagan first became involved during a campaign trip in 1980 to Daytop Village in New York City. She recalls feeling impressed by a need to educate the youth about drugs and drug abuse. Upon her husband's election to the presidency, she returned to Daytop Village and outlined how she wished to help educate the youth. Nancy Reagan's longtime Chief of Staff James Rosebush helped her expedite what she viewed as one of her legacies as First Lady. She stated in 1981 that her best role would be to bring awareness about the dangers of drug abuse:

Understanding what drugs can do to your children, understanding peer pressure and understanding why they turn to drugs is ... the first step in solving the problem.

In July 1984, Nancy Reagan visited Longfellow Elementary School in Oakland, California, to counsel schoolchildren against drug use. When asked by a schoolgirl on how to respond if offered drugs, Reagan responded "just say no," but the phrase initially went unnoticed. In January 1985, the school reviewed footage from Nancy Reagan's visit, and a 12-year-old student named Nomathemby Martini suggested that the district start a Just Say No club. Under the leadership of community organizers Tom Adams and Joan Brann, Just Say No clubs spread nationwide. In a September 1986 televised address, Nancy and Ronald Reagan credited the initial Oakland organization for popularizing the slogan, which the White House passed onto the National Institute on Drug Abuse to publish marketing developed by the advertising firm DDB Worldwide and the Ad Council nonprofit. After the White House began licensing the slogan to Procter & Gamble for branded public service announcements, Adams and Brann resigned and joined Representative Charles Rangel in criticizing the commercialization of the organization.

When asked about her efforts in the campaign, Nancy Reagan said: "If you can save just one child, it's worth it." She traveled throughout the United States and several other nations, totaling over 250,000 miles (400,000 km). Nancy Reagan visited drug rehabilitation centers and abuse prevention programs. With the media attention that the first lady received, she appeared on television talk shows, recorded public service announcements, and wrote guest articles. By the autumn of 1985, she had appeared on 23 talk shows, co-hosted an October 1983 episode of Good Morning America, and starred in a two-hour PBS documentary on drug abuse.

The campaign and the phrase "Just Say No" made their way into popular American culture when television series such as Diff'rent Strokes and Punky Brewster produced episodes centered on the campaign. In 1983, Nancy Reagan appeared as herself on Diff'rent Strokes to garner support for the anti-drug campaign. She participated in a 1985 rock music video "Stop the Madness" as well. She even appeared in numerous public service announcements, including one which aired in movie theaters where she appeared alongside actor Clint Eastwood. La Toya Jackson became spokesperson for the campaign in 1987 and recorded a song titled "Just Say No" with British hit producers Stock/Aitken/Waterman.

In 1985, Nancy Reagan expanded the campaign internationally. She invited the First Ladies of 30 nations to the White House in Washington, DC, for a conference entitled the "First Ladies Conference on Drug Abuse". She later became the first First Lady invited to address the United Nations.