Profile: Pope Francis The 1st
Born in Argentina, Pope Francis is the first Latin American to lead the Roman Catholic Church, as well as the first Jesuit.
"It seems my brother cardinals went almost to the end of the world [to choose a pope]," he told the crowd in St Peter's Square in his first address, a joke which belied his image as the cardinal who never smiles.
Up until 13 March, he was Cardinal Jorge Mario Bergoglio of Buenos Aires.
Pundits did not see him as a favourite for the job of succeeding Benedict XVI and his advanced age - at 76, he is just two years younger than Benedict at the time of his election in 2005 - may surprise those expecting a younger man as the 266th pope.
However, he appeals to both Church conservatives and reformers, being seen as orthodox on sexual matters, for instance, but liberal on social justice.
"We live in the most unequal part of the world, which has grown the most yet reduced misery the least," he was quoted as saying by the National Catholic Reporter at a gathering of Latin American bishops in 2007.
"The unjust distribution of goods persists, creating a situation of social sin that cries out to Heaven and limits the possibilities of a fuller life for so many of our brothers."
He was born on 17 December 1936 in Buenos Aires, of Italian descent.
According to his official Vatican biography, he was ordained as a Jesuit in 1969 and went on to study in Argentina and Germany.
A bishop from 1992, he became Archbishop of Buenos Aires in 1998, and at the 2005 conclave was seen as a contender for the papacy.
His election took many by surprise in his home city, where many had thought his age ruled him out, the BBC's Marcia Carmo reports from there.
But any surprise soon gave way to the jubilant blaring of car horns on the streets.
As Cardinal Bergoglio, his sermons always had an impact in Argentina and he often stressed social inclusion, indirectly criticising governments that did not pay attention to those on the margins of society, our correspondent says.
Francesca Ambrogetti, who co-authored a biography of him, told Reuters news agency that part of his public appeal lay in his humble lifestyle.
"His lifestyle is sober and austere," she said.
"That's the way he lives. He travels on the underground, the bus, when he goes to Rome he flies economy class."
In Buenos Aires, he lived in a simple flat in the building of the Archdiocese, next door to the cathedral.
When in Rome, BBC Latin America analyst Eric Camara writes, he often preferred to keep his black robe on, instead of the cardinal's red and purple vest he is entitled to wear.
He is also said to have re-used the cardinal's vest used by his predecessor.
According to Ms Ambrogetti, he is a moderate in all things.
"He is absolutely capable of undertaking the necessary renovation without any leaps into the unknown," she said.
"He would be a balancing force. He shares the view that the Church should have a missionary role, that gets out to meet people, that is active.... a church that does not so much regulate the faith as promote and facilitate it."
However, it appears that few who know him doubt his conservative credentials.
This is how Monsignor Osvaldo Musto, who was at seminary with him, described him in a BBC News article back in 2005: "He's as uncompromising as Pope John Paul II, in terms of the principles of the Church - everything it has defended regarding euthanasia, the death penalty, abortion, the right to life, human rights, celibacy of priests."
But for the Church establishment, it will be a novelty to have a Jesuit in charge: members are supposed to avoid ecclesiastical honours and serve the Pope himself.
One issue he may have to contend with is his role under the Argentine military dictatorship of 1976-1983, and particularly the abduction of two Jesuits whom the military government secretly jailed for their work in poor neighbourhoods.
The two men survived five months of imprisonment and their superior has been accused of withdrawing protection from them. It is a charge his office flatly denied.
The new pope's supporters say that, on the contrary, he helped dissidents escape from the junta.
During Argentina's economic crisis of 2001, he protested at police brutality during the unrest which saw President Fernando de la Rua swept from power.
A more immediate issue for the Vatican is the state of the new pope's health.
For decades, he has lived with only one functioning lung, although he is said to be in good shape.
He is said to be a football fan, supporting Buenos Aires team San Lorenzo de Almagro.