"I cannot ethically defend the constitutionality of Pennsylvania's version of DOMA where I believe it to be wholly unconstitutional," Pennsylvania Attorney General Kathleen G. Kane said at a press conference.
She noted that she could authorize another state official to defend the state in litigation.
Pennsylvania General Counsel James D. Schultz said that his office "was surprised" that Ms. Kane, "contrary to her constitutional duty," would decide not to defend a lawfully enacted law "merely because of her personal beliefs."
Mr. Schultz said his office had not received formal notification of her decision yet, but would be reviewing the matter.
Ms. Kane's announcement mirrors that of U.S. Attorney General Eric H. Holder Jr., who, in concert with President Obama, refused to defend the 1996 Defense of Marriage Act (DOMA), which defined marriage for the purposes of federal law as only the union of one man and one woman.
A lesbian widow sued to overturn that provision of DOMA, and in June, the U.S. Supreme Court struck it down, saying that refusal to recognize gay couples legally married under state law was unconstitutional.
On Monday, 10 same-sex couples and a lesbian widow filed a lawsuit against Pennsylvania Gov. Thomas W. Corbett and other state officials, including Ms. Kane, for enforcing Pennsylvania's marriage DOMA.
Exclusion from marriage "undermines the plaintiff couples' ability to achieve their life goals and dreams, threatens their mutual economic stability, and denies them a 'dignity and status of immense import,'" said the American Civil Liberties Union complaint, citing language from its recently won U.S. Supreme Court case, United States v. Windsor.
The Pennsylvania case, Whitewood v. Corbett, filed in U.S. District Court for the Middle District of Pennsylvania, asks that Pennsylvania's DOMA law also be overturned under the 14th Amendment's due process and equal protection clauses.
Michael Geer, president of Pennsylvania Family Institute, said he and his allies believe Mr. Corbett, who was elected governor in 2010 is on record as supporting Pennsylvania's marriage law, "will vigorously defend state law" and uphold marriage as "the unique and vital institution that benefits society and children by bringing together moms and dads."
If the ACLU thinks public opinion is so strong for gay marriage in Pennsylvania, "why not go through the political process," where most significant policy decisions are made, Mr. Geer said. Instead, they are "short-circuiting the process and trying to get a federal judge to impose this redefinition [of marriage] on Pennsylvania, no matter what the people may think."
Separately on Tuesday, longtime gay-marriage advocate Evan Wolfson said there is "irrefutable momentum" to bring gay marriage throughout America, but it will take a lot of public education and work in each state.
Gay-marriage advocacy groups will work "hand in glove" with litigation groups like the ACLU and Lambda Legal, Mr. Wolfson. But there is also a need for "education on the ground," he said, noting that Freedom to Marry, the marriage-equality group he founded, is already investing $500,000 in state marriage campaigns.
The first states to be targeted for gay marriage are Illinois, New Jersey, Oregon and Hawaii. After that, gay-marriage campaigns are likely to be held in Arizona, Colorado, Michigan, Nevada, New Mexico, North Carolina, Ohio, Pennsylvania and Virginia, he said.
Pennsylvania's DOMA, enacted in 1996, says marriage is a "civil contract by which one man and one woman take each other for husband and wife." It does not permit recognition for marriages between persons of the same sex from other jurisdictions.
Ms. Kane, a Democrat, was elected in 2012 with wide support, partly because she promised to review how the attorney general's office handled the Jerry Sandusky sex scandal.