Kane, a Democrat, told a news conference in Philadelphia the Pennsylvania Constitution forbids discrimination "against any person."
"I cannot ethically defend the constitutionality of Pennsylvania's version of" the Defense of Marriage Act, she said. "I believe it to be wholly unconstitutional."
The U.S. Supreme Court ruled last month the federal Defense of Marriage Act is unconstitutional.
The American Civil Liberties Union filed the lawsuit in Harrisburg this week with 23 plaintiffs, including 10 same-sex couples, The New York Times reported.
Republican Gov. Tom Corbett's general counsel is expected to handle the state's defense in the lawsuit, the Times said.
Rob Gleason, chairman of the Republican Party of Pennsylvania, accused Kane of "blatantly politicizing the highest law enforcement office in our Commonwealth," The Philadelphia Inquirer said.
"What law will she ignore next?" Gleason said.
Kane, who was elected last year, supported same-sex marriage during her campaign, the Inquirer said.
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