"May peace be with all of you," he says from the Vatican balcony.
Chicago-born Prevost is seen as a reformer and worked for many years as a missionary in Peru before being made an archbishop there.
Huge crowds in St Peter's Square welcome Pope Leo XIV
A cardinal appeared on the balcony in St Peter’s Square to announce that American Robert Prevost, who will be known as Pope Leo XIV, had been chosen by the votes of the Conclave to be the next pontiff.
Shortly after, the new pope appeared on the balcony to cheers from the crowds filling St Peter's Square.
The 69-year-old Chicago-born pontiff delivered his first speech, bringing a "greeting of peace" and God's love to the faithful below him.
He paid tribute to his predecessor, and even spoke briefly in Spanish to thank his former diocese.
“Help us, and help each other, build bridges,” he told the crowd of more than 40,000 people in St Peter’s Square.
A deep hum rose in St Peter's Square while reciting Ave Maria
The new Pope addressed the square in confident Italian, switching to Spanish briefly at the end when thanking his former diocese in Peru.
“I would like this greeting of peace to reach all your hearts and families… and people around the world. May peace be with you,” Pope Leo XIV began as the square fell silent.
At other moments, his address was met with frequent warm applause, especially when the new Pope mentioned peace and the late Pope Francis.
He also insisted on the need for unity, and at the end asked everyone to join together in prayer.
When he began reciting Ave Maria, a deep hum rose as the square followed suit, with some signing in their own languages.
Pope Leo is seen as someone who can bridge different worlds
There had been talk that cardinals felt the new pope would need to be someone who would take forward Pope Francis's legacy of reaching out to those on the margins, but also bring along with him a wide spectrum of those within the Church, including those Pope Francis was sometimes at loggerheads with.
Going into conclave, Cardinal Robert Prevost was thought of by observers as a candidate who could fulfil that role - someone who could be a bridge between different worlds.
The fact the conclave came to a decision in just four rounds of voting suggests the cardinals agreed with that assessment.
Leo XIV concludes first speech as Pope
Switching back to Italian, the new pontiff called on the crowd to pray to the Virgin Mary "together" for her blessing.
He then led the people in a quick prayer, which was greeted by further cheering.
After concluding his speech, the new Pope started reading a religious passage in Latin paying homage to the saints and the Madonna.
Pope Leo XIV says he wants to deliver his own blessing to the people
The new pope continued with a message of peace as he notes "God loves us all, unconditionally".
He paid tribute to his predecessor, Pope Francis, as he says he wants to deliver his own blessing to the people as Pope Francis did in his last appearance in St Peter's Square.
"Humanity needs Christ as a bridge to be reached by God and his love. Help us, and help each other, build bridges."
'May peace be with all of you,' Pope Leo XIV tells crowds
Addressing the cheering crowd in Italian, the 69-year-old says: "Peace be with all of you.
"Brothers and sisters dearest, this is the first greeting of Christ resurrected. I would like to offer a greeting of peace to reach your families, all of you, wherever you are. May peace be with you."
Source: BBC News
