Justice Anthony Kennedy, joined four liberals, said DOMA violated the equal protection guarantee of the Fifth Amendment.
The ruling does not recognize a federal right to same-sex marriage. Instead, it reaffirms the right of states to approve same-sex marriage without federal interference.
But the landmark decision should have an impact on other disputes involving marriages of gays and lesbians.
In the underlying case, Edith Schlain Windsor, an 84-year-old New Yorker, married her same-sex partner of 40 years, Thea Spyer, in 2007 in Canada. When Spyer died in 2009 of multiple sclerosis, she left her estate to Windsor.
As executor of Spyer's estate, Windsor paid approximately $363,000 in federal estate taxes, but filed a refund claim under a federal statute that says "property that passes from a decedent to a surviving spouse may generally pass free of federal estate taxes." The Internal Revenue Service denied the claim on the ground that Windsor is not a "spouse" within the meaning of DOMA Section 3 and thus not a "surviving spouse" within the meaning of the statute.
A federal appeals court ruled in her favor.
The U.S. Supreme Court affirmed the appeals court.
In the majority opinion, Kennedy said, "DOMA is unconstitutional as a deprivation of the equal liberty of persons that is protected by the Fifth Amendment. ... By history and tradition the definition and regulation of marriage has been treated as being within the authority and realm of the separate states. Congress has enacted discrete statutes to regulate the meaning of marriage in order to further federal policy, but
DOMA, with a directive applicable to over 1,000 federal statutes and the whole realm of federal regulations, has a far greater reach. Its operation is also directed to a class of persons that the laws of New York, and of 11 other states, have sought to protect."
Instead of protecting, Kennedy said, DOMA "seeks to injure."
"DOMA seeks to injure the very class New York seeks to protect. By doing so it violates basic due process and equal protection principles applicable to the federal government. ... The Constitution's guarantee of equality 'must at the very least mean that a bare congressional desire to harm a politically unpopular group cannot' justify disparate treatment of that group," Kennedy said, citing a 1973 Supreme Court precedent.
All four conservatives dissented. Justice Antonin Scalia read his dissent from the bench, a sign of unusually sharp disagreement.
"This case is about power in several respects," Scalia said. "It is about the power of our people to govern themselves, and the power of this court to pronounce the law. Today's opinion aggrandizes the latter, with the predictable consequence of diminishing the former. We have no power to decide this case. And even if we did, we have no power under the Constitution to invalidate this democratically adopted legislation.
"The court's errors on both points spring forth from the same diseased root: an exalted conception of the role of this institution in America."
DOMA excluded same-sex spouses for purposes of more than 1,000 federal statutes and programs whose administration depends in part on individuals' marital status, and denied legally married same-sex couples benefits otherwise available to legally married opposite-sex couples under federal employment, immigration, public health and welfare, tax and other laws.
The law was defended by lawyers representing the Republican leadership of the U.S. House, since the Obama administration refused to defend it.