Octogenarians Del Martin and Phyllis Lyon took the first vows in San Francisco's ornate City Hall in a ceremony presided over by Mayor Gavin Newsom, who said California was leading a trend that would sweep the nation.
Acceptance of gay marriage has grown in the US but most states outlaw it and, overall, Americans prefer to give marriage-like rights to homosexuals under a different name.
In a November election, Californians have the option to end same-sex marriage, making the next few months a crucial test of whether gays and lesbians can convince fellow citizens that there is no going back on marriage rights for all.
Martin and Lyon, who have been together for more than 50 years, were also the first to be married in 2004, during the "Winter of Love" when about 4,000 same-sex couples exchanged vows. Courts overturned those unions but last month the state Supreme Court struck down a ban on homosexual marriage, with that ruling going into effect late on Monday.
"When we first got together, we weren't really thinking about getting married," Lyon said to a small crowd that laughed, cheered and threw red rose petals.
Southern Californian couple Robin Tyler and Diane Olson, two of the plaintiffs in the state Supreme Court battle, exchanged vows in Beverly Hills, where for years they had petitioned unsuccessfully for a marriage license.
"Here is the final ending to our beautiful story - 'And they lived happily ever after'," Tyler said.
Nearby, a supporter waived a placard that said "Finally".
Still, opponents aim to fight back in November and change the state constitution to define marriage as between a man and a woman. Conservative Christians have vowed to get voters to the polls.
