The decision will almost certainly void ordinances in Chicago and Oak Park, Ill., that forbid residents to have handguns at home.
The justices ruled in favor of the Chicago residents who wish to have guns and sent the case back to Chicago for a lower court to issue a final ruling.
Legal experts on both sides of the gun-control debate predicted the ruling would trigger more lawsuits. They disagreed, however, about whether it would lead to gun laws being struck down, beyond a city's total ban on handguns.
"This case is only the beginning of the debate. Gun ownership rights — like free-speech rights — are not absolute, and state and local governments retain the authority to enact reasonable gun-control laws," said Rick Garnett, a law professor at the University of Notre Dame.
Typically, residents can have a rifle or shotgun in any state without a permit. The main exceptions are for people with a felony record or who are mentally ill, and the Supreme Court has already said those "reasonable regulations" can stand.
Seven states, including California, forbid assault weapons or semiautomatic weapons, the court noted. Those restrictions will face a legal challenge, gun rights advocates said.
The National Rifle Assn. also said it planned to focus on how some laws were enforced. For example, California authorizes residents to carry concealed weapons in public with permits. But NRA lawyers say that police in Los Angeles and San Francisco rarely issue permits, and they want to sue to challenge that practice.
"You will see lots of lawsuits, but the vast majority of laws are likely to be upheld," said UCLA law professor Adam Winkler, an expert on the 2nd Amendment. "The only thing we know for sure is that it's unconstitutional to prohibit possession of a handgun at home."
Though the high court split along ideological lines, the ruling won bipartisan praise in Washington, where Democrats have increasingly shied away from advocating gun control. Senate Majority Leader Harry Reid (D-Nev.) and Judiciary Committee Chairman Patrick J. Leahy (D-Vt.) joined Republicans in support of the outcome.
For their part, the justices are deeply split on both the meaning of the 2nd Amendment and wisdom of gun-control measures.
Justice Samuel A. Alito Jr., who wrote the opinion for the majority, said crime data from the Chicago Police Department "reveal that the city's handgun murder rate has actually increased since the ban was enacted and that Chicago residents now face one of the highest murder rates in the country."
In dissent, Justice Stephen G. Breyer said firearms "cause well over 60,000 deaths and injuries in the United States each year. Gun regulation may save lives. Some experts have calculated, for example, that Chicago's handgun ban has saved several hundred lives, perhaps close to 1,000, since it was enacted in 1983."
Two years ago, in a case from Washington, D.C., the court declared for the first time that the 2nd Amendment protects the gun rights of individuals. But the district is a federal city and not a state, and the ruling did not make clear whether state laws or city ordinances were affected.
In Monday's McDonald vs. Chicago decision, Alito explained that in the 20th century, the Bill of Rights was extended to apply to states and cities. There is no reason to leave out the 2nd Amendment, he said. Chief Justice John G. Roberts Jr. and Justices Antonin Scalia, Anthony M. Kennedy and Clarence Thomas joined to form the majority.
Alito repeated the court's earlier assurance that "reasonable regulations" of firearms can stand: "Despite the doomsday predictions" of the city's lawyers, extending the 2nd Amendment "does not imperil every law regulating firearms."
The dissenters said they feared the ruling would have a severe and dangerous effect across the country. On his final day as a justice, John Paul Stevens said he continued to believe the 2nd Amendment was "adopted to protect the states from federal encroachment," not to undercut their gun laws. Justices Ruth Bader Ginsburg and Sonia Sotomayor joined Breyer's even longer dissent.
The court did not explain why it stopped short of striking the Chicago ordinance. But in the last two years, a federal judge and the U.S. court of appeals said that high court precedents barred them from striking down the Chicago ordinance. Now that those precedents have been set aside, the high court may have thought the judges in Chicago deserved the chance to rule directly on the constitutionality of the city's handgun ban.
Outside the court, NRA head Wayne LaPierre said it was a "monumental day." He said the NRA "will not declare victory until any law-abiding citizen can go out, buy a firearm, own a firearm, protect themselves with it and use it for any other lawful purpose."
But Paul Helmke, president of the Brady Campaign to Prevent Gun Violence, told reporters the ruling was not a surprise and set a "very narrow" definition of protected gun rights.
Alan Gura, the attorney for lead plaintiff Otis McDonald in the case, said, "People very, very soon in Chicago will be able to buy and lawfully possess handguns."
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