Cardinal Timothy Dolan, the archbishop of New York, will deliver the closing prayer at the Republican Party's national convention in Florida, after Mitt Romney accepts the party's presidential nomination next week.
Mr Romney revealed the news in an interview with the Roman Catholic broadcaster EWTN that is to be broadcast later this evening.
The host of the program, Raymond Arroyo, shared excerpts from the interview last night.
Joseph Zwilling, spokesman for the Archdiocese of New York, said Cardinal Dolan's agreement to participate, which occurred "within the last two weeks," should not be seen as partisan.
"Cardinal Dolan is going to pray, not to engage in partisan politics," Mr Zwilling said. "He made it clear when he accepted the invitation that he would also accept an invitation from the Democratic national committee to offer a prayer at their convention, should they ask."
"He is going simply to pray, which is part of what a priest should do. Before accepting the invitation, Dolan told the convention organisers that it was standard church practice for the local bishop of the area to give the blessing."
But, Mr Zwilling said, "they said we would really like you to do it," so he checked with Robert Nugent Lynch, the bishop of St Petersburg, Florida., and he had no objection.
Asked in the interview about the opposition of Catholic bishops to the Obama administration's health care mandates on birth control, Mr Romney said: "Well, first of all, I'll continue to meet with Cardinal Dolan, who, by the way, is going to offer the benediction on the last evening of the Republican Convention after my acceptance speech.
"So I am making it very clear that the interest of religious freedom is something I support wholeheartedly."
Cardinal Dolan and Wisconsion representative Paul Ryan, who is Mr Romney's pick for vice president, are close; they were good friends during the years that the cardinal served as archbishop of Milwaukee, from 2002 to 2009.
New York Times