The amendment says the diocese will prevail "in cases where the provisions of the constitution and canons of the Church of the Diocese of Pittsburgh speak to the contrary" or where resolutions of the Episcopal Church USA are found "to be contrary to the historic faith and order of the one holy catholic and apostolic church."
A second vote by clergy and parishioners at the Pittsburgh Diocese's annual convention next year is required before the amendment can take effect.
Church leaders in the Pittsburgh Diocese earlier approved resolutions declaring "null and void" the national church's decisions this summer both to consecrate Bishop V. Gene Robinson of New Hampshire, who is openly gay, and to approve the blessing of same-sex unions.
Both that vote and the vote Saturday are part of a national drive among conservative dioceses and clergy seeking to distance themselves from the Episcopal Church USA for making moves they believe go against the worldwide Anglican Communion.
The national denomination of the Episcopal Church, with 2.3 million members, is the U.S. branch of the 77 million-member Anglican Communion. The Pittsburgh Diocese represents about 20,000 Episcopalians.
U.S. conservatives who believe gay sex violates Scripture have said they want Archbishop of Canterbury Rowan Williams, the spiritual head of the Anglican Communion, to authorize a separate Anglican province for them in North America.
Last week, bishops overseas announced they were in a "state of impaired communion" with the Episcopal Church, a step short of declaring a full schism. International church leaders are not expected to announce any sort of permanent break until after a commission formed by Williams reports next year on whether a split can be averted.