When Meg Whitman ran for California governor as a Republican three years ago, she said marriage was "between a man and woman" and voiced her support for Proposition 8.
But after what she called "a period of careful review and reflection," Whitman has joined the legal fight to overturn the gay marriage ban approved by California voters in 2008.
She is one of dozens of Republicans who have signed a legal filing making a conservative case for Supreme Court justices to strike down the law. The court is scheduled to hear the case next month.
The legal filing was first reported by the New York Times.
Whitman, now chief executive of Hewlett-Packard, outlined her decision in a blog post, saying marriage leads to stronger communities and better homes for children.