The singer-songwriter has recorded a video pledging her support for the cross party and cross media group in support of gay marriage.
"I'm really proud to be backing the out for marriage campaign," Faith said.
"I don't think it matters whether you're white, black, Asian, straight, gay, bisexual, hermaphrodite, tranny, diva."
She added: "Whatever you are, you have the right to get married.
"I stand strongly behind that. I think that everyone deserves to be equal and that is why I'm supporting the out for marriage campaign."
US president Barack Obama recently expressed his support for gay marriage in an interview on ABC News.