The justices said they will review a federal appeals court ruling that struck down Prop. 8, though on narrow grounds. The San Francisco-based appeals court said the state could not take away the same-sex marriage right that had been granted by California's Supreme Court.
"Today's decision by the U.S. Supreme Court to consider marriage equality takes our nation one step closer to realizing the American ideal of equal protection under the law for all people," Attorney General Kamala D. Harris said in a statement. "For justice to prevail, Proposition 8 must be invalidated so that gay and lesbian families are finally treated with equality and dignity."
The court also will decide whether Congress can deprive legally married gay couples of federal benefits otherwise available to married people. A provision of the federal Defense of Marriage Act limits a range of health and pension benefits, as well as favorable tax treatment, to heterosexual couples.
"Whatever the outcome, it's a very exciting day to have the court looking at our families and rights under the law and our equal dignity and worth," said Tara Borelli, attorney for Lambda Legal, a gay rights organization.
The Supreme Court is heavily divided, and the swing vote may come down to Justice Anthony Kennedy, a native Californian and Reagan appointee.
"I expect four or five justices to uphold Proposition 8. Why? Because the constitution of the United States doesn't have marriage in it," said Randy Thomasson, a spokesman for SaveCalifornia.com, which opposes gay marriage. "The 10th Amendment says what's not in the federal powers belongs to the states."
The high court could start hearing arguments as early as March and could make a ruling by June.
Gay marriage is legal, or will be soon, in nine states - Connecticut, Iowa, Maine, Maryland, Massachusetts, New Hampshire, New York, Vermont, Washington - and the District of Columbia.
The Associated Press contributed to this report.