At the end of an audience at the Vatican, Mr Castro said he had thanked the Pope for his contribution for the historic rapprochement.
Secret negotiations to put an end to more than five decades of hostilities were carried out inside the Vatican.
The Pope will visit Cuba on his way to the US in September.
“I am very happy. I have come here to thank him for what he has done to begin solving the problems of the United States and Cuba,” said Mr Castro.
The communist leader had stopped at the Vatican after attending Russia’s World War Two Victory Day in Moscow.
After the audience with the Pope, Mr Castro said he was so impressed by a Vatican audience with Pope Francis that he might return to the faith he was born into.
Mr Castro praised the pontiff’s wisdom, adding: “I will resume praying and turn to the Church again if the Pope continues in this vein.”
The Catholic Church has maintained ties with Havana since the 1959 revolution.
Analysis: Will Grant, BBC correspondent, Havana
The fact that the man who helped lead the Cuban Revolution would even joke about returning to the Catholic Church shows just how far the relationship between Havana and the Vatican has moved forward recently.
That has been particularly true under Pope Francis. First, the Pontiff played a crucial role in smoothing the path to negotiations between Cuba and the US over the past 18 months. Furthermore, he has given his blessing to the process and to the Cuban government by arranging to come to Cuba in September before his visit to the US.
As a Latin American himself, Pope Francis has always been able to maintain good ties with Latin American leaders from both the left and the right. He has repeatedly called for the US trade embargo on Cuba to be lifted, for example.
Now he has hosted, and seemingly wooed, Raul Castro in Rome strengthening the ties even further. Hardly surprising Mr Castro is considering returning to Mass!
But the state-run newspaper Granma omitted Mr Castro’s comments about returning to the Church when it reported the meeting on its website. A reflection, perhaps, of how surprising it is for Cubans to hear Mr Castro make such comments, whether tongue-in-cheek or not.
For Pope Francis, the restoration of relations between the US and Cuba has been a major diplomatic achievement, says the BBC’s David Willey in Rome.
The US had imposed a trade embargo after Cuba’s revolution, which it began to lift late last year.
After the 50-minute private audience on Sunday, Mr Castro told reporters: “The pontiff is a Jesuit, and I, in some way, am too. I studied at Jesuit schools.”
After suggesting he might turn again to the Church, he added: “I mean what I say.”
Both Mr Castro and his brother, revolutionary leader Fidel Castro, were baptised as Roman Catholics, but most Church activities were suppressed after the revolution.
Francis will be the third Pope to travel to Cuba, following visits there by John Paul II in 1998 and Benedict XVI in 2012.