The Washington National Cathedral, where the US gathers to mourn tragedies and celebrate new presidents, will soon begin performing same-sex marriages.
Cathedral officials said the church will be among the first Episcopal congregations to implement a new rite of marriage.
The church will announce its new policy today.
As the nation's most prominent church, the decision carries huge symbolism.
The 106-year-old cathedral has long been a spiritual centre for the nation, hosting funerals for Ronald Reagan and Gerald Ford. It draws hundreds of thousands of visitors.
The Very Reverend Gary Hall, the cathedral's dean, says the church is taking "the next step for marriage equality".
He says including gays and lesbians is "being faithful to the kind of community that Jesus would have us be".