The National Minimum Drinking Age Act of 1984 (23 U.S.C. § 158) was passed by the United States Congress and was later signed into law by President Ronald Reagan on July 17, 1984. The act would punish any state that allowed persons under 21 years to purchase alcoholic beverages by reducing its annual federal highway apportionment by 10 percent. The law was later amended, lowering the penalty to 8 percent from fiscal year 2012 and beyond.Despite its name, this act did not outlaw the consumption of alcoholic beverages by those under 21 years of age, just their purchase. However, Alabama, Indiana, Kansas, Michigan, North Carolina, Pennsylvania, and Vermont, and the District of Columbia extended the law into an outright ban. The minimum purchase and drinking ages is a state law, and most states still permit "underage" consumption of alcohol in some circumstances. In some states, no restriction on private consumption is made, while in other states, consumption is only allowed in specific locations, in the presence of consenting and supervising family members, as in the states of Colorado, Maryland, Montana, New York, Texas, West Virginia, Washington, Wisconsin, and Wyoming. The act also does not seek to criminalize alcohol consumption during religious occasions (e.g., communion wines, Kiddush).

The act was expressly upheld as constitutional in 1987 by the United States Supreme Court in South Dakota v. Dole.