Pennsylvania's ban on gay marriage was overturned by a federal judge Tuesday.
U.S. District Judge John E. Jones III called the plaintiffs -- a widow, 11 couples and one of the couples' two teenage daughters -- courageous.
"We now join the 12 federal district courts across the country which, when confronted with these inequities in their own states, have concluded that all couples deserve equal dignity in the realm of civil," Jones wrote.
An appeal to the 3rd U.S. Circuit Court of Appeals is likely. Gov. Tom Corbett's office had defended the law after Attorney General Kathleen Kane called it unconstitutional and refused to defend it.
State marriage bans have been falling around the country since the U.S. Supreme Court last year struck down part of the federal Defense of Marriage Act.
In all, 18 states give legal status to gay marriage. If Jones' decision stands, Pennsylvania would become the 19th and legalize gay marriage throughout the Northeast.
Lawyers for the plaintiffs say the law inflicts harm on same-sex couples and their children by depriving them of the legal protections and tax benefits afforded to married couples.
The Pennsylvania lawsuit, filed July 9, was the first known challenge to the state law that effectively bans same-sex marriage and the recognition of gay marriages from other states.
At least five later challenges have surfaced in state and federal courts since the lawsuit was filed, including one in which a county official is defending his decision to issue 174 marriage licenses to same-sex couples.
Oregon became the 18th state to recognize same-sex marriage on Monday, when jubilant couples began applying for marriage licenses immediately after U.S. District Judge Michael McShane issued a ruling that invalidated that state's voter-approved same-sex marriage ban.
Also Monday, a federal judge in Utah ordered state officials to recognize more than 1,000 gay marriages that took place in the state over a two-week period before the U.S. Supreme Court halted same-sex weddings with an emergency stay.