"Raul Castro thanked the Pope for his mediation between Cuba and the United States," said Vatican spokesman Federico Lombardi of the exchange that also focused on Francis' upcoming visit to Cuba.
The first South American pope played a key role in secret negotiationsbetween the United States and Cuba, which led to the surprise announcement inDecember that the two countries would seek to restore diplomatic ties after more than 50 years of tensions.
During the audience Castro offered the pontiff a painting by Cuban artistKcho inspired by the plight of illegal immigrants stranded at sea.
The pope in turn presented Castro a medal of Martin de Tours, a Frenchsaint celebrated for having given his coat to a beggar, and urged others to "clothe and support the poor."
Castro, who was accompanied by his Foreign Minister Bruno Rodriguez Parrilla, held what was scheduled as a "strictly private" meeting with the pontiff in a small room adjoining the Paul VI Audience Hall, where large gatherings are held in the Vatican.
Pope Francis arrived ten minutes ahead of Castro.
A dozen unformed Swiss Guards stood to attention in front of the buildingwhen the limousine bearing the Cuban flag arrived.
The Holy See has revealed the Argentine pope personally mediated betweenthe US and Cuba, and that the Vatican hosted delegations from the two countries in October.
US theologian Miguel Diaz, a former ambassador to the Holy See, saidFrancis would reprise the words of late Polish pope John Paul II, who made ahistoric first papal visit to Cuba in 1998.
"Let Cuba open itself to the world, and let the world open itself to Cuba,"John Paul II urged during the visit, when he was accompanied by Jorge Bergoglio, then auxiliary bishop of Buenos Aires and now Pope Francis.
The pope will "certainly reiterate" John Paul II's message, "now that Cubais trying to step up its involvement in the economic world and international relations," Diaz told the Italian news agency Adnkronos.
Castro's Vatican visit, announced only Tuesday, follows a visit to Russia,where the Cuban leader attended a grandiose World War II victory parade onSaturday.
He will meet Italian Prime Minister Matteo Renzi in Rome later on Sunday.
The Vatican announced last month that Pope Francis would visit Cuba inSeptember, becoming only the third pontiff to do so after John Paul II in 1998 and Benedict XVI in 2012.
Francis will go on from Cuba to the US for a meeting with President BarackObama.
Castro's ailing older brother Fidel came to the Vatican in 1996 when he metpope John Paul II.
The Catholic Church has consistently backed calls for the lifting of the UStrade embargo against Cuba, while staunchly supporting Cuban Catholics andpressuring Havana to release political prisoners, many of whom are Catholicactivists.
The Vatican has also kept its distance from Cuban exiles based in Miami,Florida, who have long clamoured for Havana's Marxist regime to be ousted.
When the now retired Benedict XVI visited the Caribbean island he hadlengthy, warm talks with Fidel Castro, who is now 88.
The Vatican's mediation between Cuba and the US administration was adiplomatic success for the Holy See and had a considerable impact in mainlyCatholic Latin America.
Other diplomatic efforts have been less successful, including a bid to help resolve the political crisis in Venezuela and a longstanding drive to encourage reconciliation between the Colombian government and guerrilla movements in that country.