NEW YORK (Reuters) - A law that defines marriage as between a man and a woman was found unconstitutional by a federal judge in New York on Wednesday because it improperly interferes with states' rights to regulate marriage.

The ruling by Manhattan federal court judge Barbara Jones followed a decision last week by the U.S. Court of Appeals for the 1st Circuit in Boston that concluded the Defense of Marriage Act discriminates against gay couples.

Judge Jones became the fifth judge to find the 1996 law unconstitutional, adding weight to the demands of law makers and activists who want the law repealed.

On May 9, Barack Obama became the first president to publicly express support for gay marriage, saying he believes same-sex couples should be allowed to marry.

On Wednesday, Judge Jones said that under the pretext of creating a uniform law for all, the law intrudes "upon the states' business of regulating domestic relations."

"Such a sweeping federal review in this arena does not square with our federalist system of government," Jones wrote.

Eight of the 50 U.S. states permit gay marriage, including Massachusetts, which became the first in 2004, and Maryland, the most recent.

On Tuesday, a federal appeals court in San Francisco declined to reconsider its decision tossing out a California ban on gay marriage, clearing the way for the Supreme Court to consider whether the ban violates the U.S. Constitution.

Wednesday's case was brought against the United States in 2010 by Edith Windsor, who sued to recover $353,053 in federal estate tax she was not allowed to claim because her marriage to Thea Spyer, who died in 2009, was not legally recognized. Windsor and Spyer married in Canada in 2007.

The judge ordered the money to be returned to Windsor.

"It's a good day for justice," said Windsor's attorney Roberta Kaplan.

After the Obama administration said last year that it considered the law unconstitutional and would no longer defend it, the Bipartisan Legal Advisory Group of the U.S. House of Representatives (BLAG) took up its defense.

The law prohibits same-sex couples from receiving marriage-based benefits such as Social Security survivor benefits, health benefits and jointly filing taxes.

BLAG attorney Paul Clement did not immediately respond to a request for comment on Wednesday's ruling.