The Rev. Federico Lombardi, a Vatican spokesman, told reporters the two discussed religion and world affairs in a "very familiar, very informal" meeting in Havana, following a sermon attended by thousands of Cubans.
Castro is reported to have given the Pope a book titled "Fidel & Religion: Conversations with Frei Betto on Marxism and Liberation Theology," while Francis gave Castro three books, including a collection of sermons by Castro's former teacher.
Beforehand, more than 300,000 Cubans attended the Pope's homily at the Plaza de La Revolución, in which he advocated Christians set aside their own needs "and to look instead to those who are most vulnerable," adding that service is not about ideology, "for we do not serve ideas, we serve people."
Francis also praised improved cooperation between Cuba and the Catholic Church, as well as talks between the Colombian government and rebels with the Revolutionary Armed Forces of Colombia, or FARC, which have gone on in Havana since 2012.
Pope Francis, who has been credited with assisting in a thaw in U.S.-Cuban relations, is scheduled to begin his trip to the United States on Tuesday.