The decision by the seven-judge Court of Appeals leaves in place a unanimous lower-court ruling that senators and gay-marriage backers did not violate the state's open-meetings law when they met behind closed doors as the bill moved to the floor.
The evangelical Christian group New Yorkers for Constitutional Freedoms had argued the Senate didn't follow proper procedures by meeting privately.
"With the court's decision, same-sex couples no longer have to worry that their right to marry could be legally challenged in this state," Gov. Andrew Cuomo, who pushed for passage of the law last year, said in a statement.
"The freedom to marry in this state is secure for generations to come," he said.
New Yorkers for Constitutional Freedoms Executive Director Rev. Jason McGuire said in a statement his group was "disappointed but not surprised by today's decision.
"Every time the people of a state have had opportunity to vote on this issue, they have rejected same-sex 'marriage,' but when rogue legislators and activist courts get involved they reject the will of the people," said his statement, posted on the group's website.
"What is most troubling is that the court has surrendered its rightful role as a check and balance on an out-of-control Legislature," he said.
State lawmakers voted 33 to 29 to legalize same-sex marriage June 24, 2011, making New York the largest state where gay and lesbian couples became able to wed.
Four members of the Republican majority joined all but one Democrat in the Senate in supporting the state's Marriage Equality Act after an intense and emotional campaign.
The measure had been approved by the state Assembly a week earlier.
Cuomo signed the bill into law at 11:55 p.m., minutes after the Senate vote, and it went into effect 30 days later.
Cuomo, a Democrat, Thursday endorsed Republican state Sen. Stephen Saland of Poughkeepsie, who cast the decisive vote for the measure. Saland voted "no" on same-sex marriage in December 2009.