Del Martin and Phyllis Lyon, both aged over 80, took the first vows in San Francisco's City Hall in a ceremony presided over by Gavin Newsom, the mayor.
The two, 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, although courts overturned those particular unions.
"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.
Newsom predicted same-sex marriages would be legalised in other states across the US.
Marriage as an institution has been strengthened," he said.
A recent University of California Los Angeles (UCLA) study said around 51,000 of the 102,000 same-sex couples living in California are expected to marry over the next three years.
A further 67,500 same-sex couples from outside the state expected to wed here during the same period.
The survey published last month said 52 per cent of Californians supported homosexual marriage while 41 per cent opposed it.
Bill Bradley, a California political analyst, told Al Jazeera that the issue was unlikely to have a big impact on the forthcoming US presidential elections.
Bradley said Arnold Schwarzenegger, the state's Republican governor, was likely to campaign against any new moves to ban same-sex marriages.
Both presumptive candidates for the presidency, John McCain and Barack Obama, support civil unions for same-sex couples but stop short of supporting actual marriage.
In the US, also Vermont and Connecticut recognise same-sex civil unions, while Hawaii, Maine and New Jersey allow couples living together the same rights as married couples.
Last month's California Supreme Court court ruling came after a seven-year legal battle that erupted in 2000 when the state's voters approved a law that only marriages between men and women could be legally recognised.
However civil rights activists argued that limiting marriage to opposite-sex couples was unconstitutional and that the law should be struck down.
|Religious campaigners have targeted|
gay rights activists around the world [EPA]
In 2005 the San Francisco Superior Court ruled in favour of those opposed to the ban, finding that there was no justification for refusing to allow the marriages.
But the decision was overturned in 2006 by the California Court of Appeal, which ruled in a 2-1 decision that the state's desire to "carry out the expressed wishes of a majority" was sufficient to preserve the existing law.
Holland was the first country to allow gay marriages in April 2001, followed by Belgium in June 2003, Canada and Spain in July 2005.
South Africa was the first African country to legalise same-sex marriage after it passed the Civil Union Act in 2006.
But many other countries reject any kind of union between homosexuals, usually on religious grounds.
The Australian government passed legislation in 2004 that defined marriage as "a union between a man and a woman to the exclusion of all others".
In India, homosexuality is an offence punishable by a life sentence in prison, and same-sex marriages are also prohibited in China and Russia.