The amendment officially takes effect on June 21 -- one year from the church's 2014 General Assembly in Detroit, which overwhelmingly endorsed the amendment.

The Detroit assembly also had given immediate permission for pastors to conduct same-sex marriages in states where it's legal, such as Pennsylvania. But the amendment now extends that permission nationwide.

It defines marriage as a "unique commitment between two people, traditionally a man and woman" but no longer limited by gender.

The measure also says no pastor or church can be compelled to participate in a same-sex wedding.

The ratification -- combined with an amendment four years ago allowing the ordination of non-celibate gays and lesbians -- decisively breaks what had been a decades-long stalemate over homosexuality in the church.

"Our denomination steps forward into a new chapter," said the Rev. Randy Bush, pastor of East Liberty Presbyterian Church in Pittsburgh and co-moderator of the Covenant Network of Presbyterians, a national liberal advocacy group.

The vote, he said, "respects the importance of pastoral discretion in congregational matters and yet offers to the world a compelling witness about how God's grace is active in loving, mutual relationships."

But Carmen Fowler LaBerge, president of the conservative Presbyterian Lay Committee, predicted the denomination would lose members, congregations and overseas relationships.

"We regard this vote as a repudiation of the Bible and the clear teaching of the Word of God," said Ms. LaBerge. "This places the Presbyterian Church (U.S.A.) outside of the fellowship of the global Christian church. And it will be difficult going forward for the PC (USA) to have any moral authority on any other issue."

Some historic Protestant denominations -- such as the Episcopal Church and Evangelical Lutheran Church in America -- allow local congregations' discretion in whether to recognize same-sex unions. The Presbyterian action was noteworthy for writing an expanded marriage definition into its core governing document.

The changes in the 1.8 million-member Presbyterian Church (U.S.A.) come at a time of accelerating membership declines -- 10 percent in just the past two years.

More than 300 congregations, dozens of them in Pennsylvania, have reacted to liberal trends in the past several years by leaving for smaller, conservative Presbyterian denominations.

That has left fewer conservatives voting on contentious issues at a time of growing social acceptance of gay marriage.

Several presbyteries nationwide -- including Lake Erie in northwestern Pennsylvania and Carlisle and Huntingdon in central Pennsylvania -- voted yes to marrying gays after voting no to ordaining them four years ago.

But several Western Pennsylvania presbyteries have voted no to gay marriage and to gay ordination, including Beaver-Butler, Redstone, Kiskiminetas, Shenango and Washington.

The Pittsburgh Presbytery has not yet voted on the marriage amendment.

Robert Gagnon, a professor of New Testament at Pittsburgh Theological Seminary who has written against church acceptance of homosexuality, said the Presbyterians had "incurred a serious self-inflicted wound."

He added: "Any denomination that eliminates a male-female requirement for marriage breaks faith with a foundational element in our Lord Jesus' sexual ethics. It breaks faith also with the apostolic witness to Christ in the New Testament, the Hebrew Scriptures that preceded Jesus, and the historic tradition of the Church universal."

The Rev. Brian Ellison, executive director of the Covenant Network of Presbyterians, said while the group is pleased with the result, "We also recognize the church is not of one mind about this issue, and we're committed to finding ways to live together faithfully."

The Rev. Gradye Parsons, stated clerk for the Louisville, Ky.-based denomination, said he hoped opponents to the change would remain in the Presbyterian Church (U.S.A.)

"The amendment mandates no congregation has to host same gender weddings so hopefully that forbearance will encourage people to stay," he said.

But Ms. LaBerge said she "would be surprised if this were not one step in the direction of compelling pastors to perform and compelling sessions to allow churches to be used for same-sex weddings."

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