"America is at its best when we champion the freedom and dignity of
every human being," Clinton said in a video produced by the Human Rights
The former first lady, U.S. senator, and 2008 presidential candidate had previously backed civil unions and partner benefits for same-sex couples, but had stopped short of a full endorsement for marriage. That was the position of most Democratic primary candidates that year, including President Barack Obama.
During her tenure as secretary of state, Clinton avoided taking political positions, as is customary for the role.
Her husband, former President Bill Clinton, wrote in an opinion article last week that he supported the repeal of the Defense of Marriage Act, the law he signed in 1996 that defines marriage as a legal union between a man and a woman. It also denies federal benefits to same-sex couples in the nine states where same-sex couples can now legally wed.
In the video released Monday, Hillary Clinton said her time traveling the world as America's top diplomat "inspired and challenged me to think anew about who we are, and the values we represent to the world."
Those values, she said, must include full equal rights for gay and lesbian Americans to marry.
"Full and equal citizens and deserve the rights of citizenship," she said. "That includes marriage. That's why I support marriage for lesbian and gay couples. I support it personally and as a matter of policy and law."
Clinton had previously hinted at her support for marriage rights, speaking favorably of New York's law permitting marriages between same-sex couples. It also brings her in line with major figures in the Democratic Party, including Obama, who came out in support of marriage rights last May.
Democrats also included a plank in their party platform at last summer's convention supporting same-sex marriage.
Clinton's public backing of same-sex marriage comes as the Supreme Court prepares to tackle the issue this spring. Last year the high court agreed to hear two constitutional challenges to state and federal laws dealing with the recognition of gay and lesbian couples to legally wed.
Oral arguments will be held on March 26 and 27, with a ruling by late June.
In her video, Clinton called marriage "a fundamental building block of our society," and with a knowing laugh, called marriage "a great joy and yes, a great responsibility."
"A few years ago, Bill and I celebrated as our own daughter married the love of her life, and I wish every parent that same joy," she said.