Lawyers for Gov. Corbett, whose administration had defended the law, said they were reviewing the decision. They have 30 days to appeal.
But advocates hailed the decision.
"What a great day!" said Mark Aronchick, one of the plantiffs lawyers in the case. "The court, in a bell-ringing opinion, has explained in crystal clear language why the promises of our Constitution extend to all Pennsylvanians. We urge the commonwealth to take whatever steps are necessary to allow marriages to proceed and the celebrations to begin immediately."
In Philadelphia, Register of Wills Ron Donatucci's office immediately began issuing marriage licenses to same-sex couples. Guy Sabelli, the city's marriage license supervisor, said 12 female couples and 6 male couples had obtained licenses by the time the office closed at 5:30 p.m. The office reopens at 8 a.m. Wednesday with extended hours until 7 p.m.
The lawsuit, Whitewood v. Wolf, was brought by 23 plaintiffs who said Pennsylvania's law violates the constitution by excluding same-sex couples from the same legal benefits and protections as heterosexual couples.
A trial was initially expected this summer. But lawyers on both sides of the case had told Jones the evidence and testimony wouldn't expand beyond what they had already presented in arguments and filings, and urged the judge rule based on them.
"The issue we resolve today is a divisive one," Jones wrote. "Some of our citizens are made deeply uncomfortable by the notion of same-sex marriage. However, that same-sex marriage causes discomfort in some does not make its prohibition constitutional. Nor can past tradition trump the bedrock constitutional guarantees of due process and equal protection."
In the 10 months since the U.S. Supreme Court struck down the federal defense of marriage act and the Pennsylvania law suit was filed, marriage bans have fallen in states across the nation.
Courts at the state and federal levels in 12 states have found in favor of same-sex marriage. Some courts overturned same-sex marriage bans, while others allowed recognition of marriages conducted in other states.
The case before Jones, a 58-year-old Republican appointed in 2002 by President George W. Bush, was one of seven same-sex marriage cases under consideration by courts in Pennsylvania.
The state Supreme Court is considering an appeal by Montgomery County Register of Wills L. Bruce Hanes to a Commonwealth Court decision barring him from issuing marriage licenses to same-sex couples.
Should Jones' ruling stand up to appeal, it would render moot at least one other lawsuit, heard in a Philadelphia courtroom last week, seeking to strike down the portion of Pennsylvania's law barring recognition of out-of-state, same-sex unions.
U.S. District Judge Mary McLaughlin has yet to rule on the case of Isabelle Barker and Cara Palladino, a lesbian couple wed in Massachusetts in 2005, who sued for recognition of their marriage license in Pennsylvania.
But the couple said Tuesday it hardly mattered to them whether it was their suit or another that accomplished their goals. They celebrated with a champagne toast in their offices at Bryn Mawr College, where both women work.
"There are a lot of people who worked on this issue to make this day come," Palladino said. "It's a great day for all Pennsylvanians. It's a great day for justice and equality in Pennsylvania, now that we've cracked the liberty bell."
In a March Quinnipiac University poll of Pennsylvania voters, 57 percent of respondents said they supported a law allowing same-sex couples to marry, while 37 percent opposed it. The poll results trended along party lines: 74 percent of Democrats supported same-sex marriage, while 59 percent of Republicans opposed it.
In the Whitewood case, lawyers for the plaintiffs alleged that Pennsylvania's Defense of Marriage Act, along with its refusal to marry same-sex couples or recognize such marriages from other states, violated a fundamental right to marry as well as the Constitution's equal-protection clause.
The state's law not only bans same-sex marriage, it specifies that the state cannot honor such marriages from other states.
The suit contended the state had no legitimate interest in banning same-sex marriage, and that the ban disparaged and injured lesbian and gay couples and their families by denying them a long list of legal and financial protections afforded to heterosexual couples.
The Pennsylvania Catholic Conference, which had supported the law, said Jones' decision "speaks to the confusion and misunderstanding among many today about the fundamental building block of society: the family. Every child has a basic right to a mother and a father united in marriage as a family. Today's decision does not change that."
Attorney General Kathleen G. Kane, whose office had declined to defend the law, hailed the groundbreaking ruling.
"This is an historic day," she said in a statement. "More importantly, today brings justice to Pennsylvanians who have suffered from unequal protection under the law because of their sexual orientation. When state-sponsored inequality exists, citizens are deprived of the full protections that the Constitution guarantees. Our Commonwealth progressed today and so have the hopes and dreams of many who suffer from inequality."
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Staff writers Jeremy Roebuck and Bob Warner contributed to this report.