The friend-of-the-court document makes Obama the latest in a list of prominent folks who have asked the U.S. Supreme Court to overturn the state's ban.
Among those who have taken a stand against Proposition 8 are prominent conservatives including Clint Eastwood, Meg Whitman and Paul Wolfowitz, big companies like Apple, Facebook and Intel, and 14 states including California.
The new legal briefs have not yet appeared on the Supreme Court website, though some earlier briefs are there, but the American Foundation for Equal Rights has published some of the new ones on its own site.
Earlier the court also received many petitions in support of Proposition 8, many of them from religious groups such as the U.S. Conference of Catholic Bishops. Some of those briefs, along with others in opposition, can be found on the American Bar Association website.
How much weight Obama's opinion will carry in the court remains to be seen.
Obama has said in the past that questions of same-sex marriage should be left up to states, so some observers, such as University of California Hastings Law Professor Rory Little, wonder if the public stance will undermine the president's credibility.
Obama appears to be moving with the tide of public opinion. A Field Poll released Wednesday found 61 percent of California voters in favor of same-sex marriage, versus 32 percent against.
The measure passed in 2008 with 52 percent of ballots cast in favor.
Opponents took their case to court, and won decisions against the measure in lower courts. The Supreme Court plans to take up Proposition 8 in March, along with the Defense of Marriage Act, a federal law banning the recognition of same-sex marriages.
News that the Obama Administration will throw its weight behind efforts to overturn California's Proposition 8 is getting strong reaction on both sides of the gay marriage debate.
San Francisco City Attorney Dennis Herrera told KQED's Scott Shafer he was "delighted" that the President was lending, in his words, "the legal and moral weight of the federal government" on behalf of same sex marriage.
But attorney Andrew Pugno with the Yes on 8 legal team calls it a significant reversal on the President's part. "Prior to the election he was very clear that he thought this was an issue that belonged to the states and that it was a healthy and good thing for this debate to work itself out at the state level," he said. "And by stepping in and attacking an individual state's marriage laws that essentially has federalized the issue."
The Obama Administration has already weighed in against the Defense of Marriage Act, which is also being heard by the Supreme Court next.
Here is a copy of the brief from the Obama Administration:
Obama Amicus Brief on Prop 8 by
