Pope Francis has met Cuba's Raul Castro who thanked the pontiff for brokering the thaw between Havana and Washington.
Mr Castro said the pope so impressed him that he might return to the Catholic Church, despite being a communist.
Raul Castro, who is the 83-year-old younger brother of Cuba's revolutionary leader Fidel, spoke with the Argentine-born pope for nearly an hour.
The Vatican said the meeting was strictly private and not a state visit.
Papal audiences on Sundays are extremely rare.
Pope Francis made an exception when Mr Castro asked if he could stop in Rome on his way back from Moscow to thank Francis for the Vatican mediation between the United States and Cuba, Cuban officials said.
Pope Francis is due to visit both Cuba and the US in September and excitement is already building in Havana ahead of his arrival.
The Church's activities in Cuba were suppressed for decades after the revolution of 1959.
The government began loosening restrictions in the early 1990s.
After the late Pope John Paul II visited in 1998, Fidel Castro re-instated Christmas as a holiday.
Leaving the Vatican after his meeting with Pope Francis, Mr Castro said he and the pontiff had a magnificent meeting.
He said he came to thank him for what he did in order start finding a solution to the problems between the US and Cuba.
The pope's US trip had been planned for some time before the Vatican announced last month that Francis would stop in Cuba on his way to Washington.
It will be the pope's first visit to both countries as pontiff.
His predecessors, John Paul II and Benedict XVI, both visited Cuba and met Fidel Castro.