Bush added her voice to a campaign called "New Yorkers for Marriage Equality," which is sponsored by the Human Rights Campaign.
In a video on the group's website, Bush says, "I'm Barbara Bush, and I'm a New Yorker for marriage equality. New York is about fairness and equality. And everyone should have the right to marry the person that they love. Join us."
She lives in Manhattan.
As president, her father, George W. Bush, pushed for a constitutional amendment banning same sex marriage.
The Human Rights Campaign, which calls itself "the nation's largest lesbian, gay, bisexual and transgender (LGBT) civil rights organization," says, "Bush's advocacy shows that equality knows no party label and raises the profile of this timely fight for equal marriage rights in the Empire State."
It continued, "Barbara Bush joins prominent Republicans like her mother, Laura Bush, as well as Dick Cheney and Ted Olson as supporters of marriage equality. Her position also reflects her generation with 68 percent of New Yorkers between the ages of 18-34 supporting marriage for same-sex couples."
The Campaign adds that a January 2011 Siena Research Institute Poll found 57 percent of New Yorkers support marriage equality. In another poll, just released by Quinnipiac University, New Yorkers support marriage equality by the highest margin ever in that poll, 56 percent to 37 percent, the Campaign says.