The Episcopal Church has become the largest US denomination to bless same-sex relationships.
The policy was overwhelmingly approved in a vote at the church's general convention in Indianapolis, Indiana.
Church officials stressed the new ceremony, which includes prayers and an exchange of vows and rings, was not a same-sex marriage.
The US Episcopal Church - part of the 77 million-member Anglican Communion - has nearly two million worshippers.
On Tuesday nearly 80% of the Episcopal Church's House of Deputies voted to authorise a three-year trial run for a provisional same-sex service.
A day earlier, the House of Bishops also resoundingly approved the new ceremony, which is called the "Witnessing and Blessing of a Lifelong Covenant".
"I believe the Episcopal Church will continue to evolve on the issue of marriage equality and look forward to joining our brothers and sisters in being a headlight instead of taillight on marriage equality," said the Rev Susan Russell, an Episcopal priest.
But opponents argued during Tuesday's debate that the service amounted to an endorsement of same-sex marriage without theological justification.
"It is being seen as a marriage rite even though I was told that is not intended," said the Rev Sharon Lewis, from a Florida diocese.
Episcopal Church law and the Book of Common Prayer define marriage as the union of a man and a woman.
Under the new policy, each Episcopal bishop will decide whether to allow the ceremonies in his or her diocese. Same-sex couples must also complete counselling before undergoing the rite.
A "conscience clause" allows Episcopalians to oppose its use without facing a penalty.
Also on Monday, the full Episcopal convention approved new anti-discrimination language for transgendered people that paves the way for transgendered clergy.
Other mainline Protestant churches have removed barriers to gay ordination in recent years or allowed individual congregations to celebrate gay or lesbian unions.
Episcopalians caused uproar in 2003 by consecrating New Hampshire Bishop Gene Robinson, the first openly gay bishop in the Anglican world.
But the move caused the Anglican Communion to begin fracturing, and it has continued to do so ever since.
Six US states and the District of Columbia have legalised gay marriage and three more states could do so this year.
Thirty US states have passed constitutional amendments limiting marriage to unions between a man and a woman.