Francis has polished his English during recent trips to Asia, but the Vatican spokesman, the Rev. Federico Lombardi, said the 78 year old Argentine pope simply finds it easier to express himself in Spanish.
He said there was no symbolic significance to the pope's decision.
Two of Francis' major speeches will be in English: his remarks at the White House and speech to Congress, the first ever by a pope.
Francis will also deliver English greetings to U.N. staff and benefactors in Philadelphia. But his homilies at a canonization in Washington, his big Mass in Madison Square Garden and at the Catholic Church's world family conference in Philadelphia will be in Spanish, as will his address to the U.N
Pope Francis ignites old debate among U.S. Catholics. He said Francis would only have a few limited occasions to greet the crowds from his open-sided popemobile.
"Maybe these aren't long stretches like we saw in the Philippines or Sri Lanka," he said. "But the Pope intends to move around as he usually does."
Francis leaves Cuba and arrives in the United States on Sept. 22 for a five-day visit.
Lombardi said security would be tight on the U.S. portion of the trip, especially in New York, since Francis' arrival coincides with the U.N. General Assembly gathering of world leaders.
Lombardi added that the Vatican was unaware of any particular threat against the Pope, and is not worried about his safety.
The Pope intends to move about as he does normally, without an armored vehicle.
House Homeland Security Committee Chairman Michael McCaul said that the U.S. has stopped at least one threat made against Francis.