Pope Leo XIV had a cancerous tumour removed from his right lung in a 90-minute operation on Tuesday and "woke up normally" after the procedure, the Vatican said.
A Vatican source said further tests were being carried out to determine if the cancer had spread.
"To ensure the procedure was radical, the lower lobe of the right lung was removed," the Vatican said in a statement.
The operation "confirmed the diagnosis of a pulmonary adenocarcinoma, as suspected from the pre-operative tests", it said.
The 71-year-old pope was in the operating room at Rome's Gemelli hospital for a total of three hours and the operation lasted half that time.
"The surgery was successful and there were no complications," the Vatican said, with a source adding that he had requested dinner after the procedure.
Juanita Mora, medical spokesperson for the American Lung Association, told AFP that when this type of cancer is "caught in early stages, then it has a very, very good prognosis".
She said much would depend on the stage of the cancer found and whether it had spread.
In cases of surgical removal and when the cancer is caught early and localised, five-year survival rates from lung adenocarcinoma were "80 to 98 percent", she said.
"They'll determine if he needs chemotherapy, radiation therapy, targeted therapy, or immunotherapy for the adenocarcinoma," she said.
'Thoughts and prayers'
US Secretary of State Marco Rubio, a practicing Catholic, said after the surgery that he and his wife were "united in prayer for (Leo's) health following his recent diagnosis".
"May the Lord give him strength and the Holy Spirit guide the medical professionals taking care of the Holy Father," Rubio said on X.
Australian Prime Minister Anthony Albanese said his "thoughts and prayers" were with the pontiff, calling him "a voice for peace, humanity and reason in our modern world".
Leo had entered the hospital smiling and waving earlier on Tuesday hours before the surgery.
He is staying in a room on the 10th floor of the building, next to a papal suite with a private chapel that is currently under renovation.
The Vatican has said it expects the recovery to take a week.
His audiences have been cancelled until October 20 and the operation raises questions about whether the pope will go on a lengthy visit to Latin America next month.
The surgery marks Leo's first hospitalisation since he became head of the Catholic Church on May 8, 2025.
In a surprise announcement following Angelus prayers on Sunday, Leo told crowds in St Peter's Square that doctors had discovered a nodule "at a very early stage" during medical checks.
"The doctors believe it would be right to intervene," he said.
The Vatican has historically been very secretive about the health of popes, but began to be more transparent during the illness of Leo's predecessor Francis, an Argentine who freely discussed his ailments.
Ahead of the operation, people outside the hospital expressed concern over the pope's condition.
Giorgia Fei, a 57-year-old Italian, said she had been worried.
"However, the Holy Father is in very good hands here at the hospital because we have some outstanding doctors," she said.
"I will pray a lot for him."
Giuseppe Purpura, 72, said he too was concerned.
"He's still quite young and I really wasn't expecting this. I thought he was in better health," he said, noting that "thankfully it was caught in time".
Latin America trip in doubt
The Vicar General for the Diocese of Rome, Cardinal Baldassare Reina, led a rosary prayer for the pope inside the hospital chapel which was broadcast by the Vatican.
Dozens of people attended, and Reina asked all the city's parishes to join in.
The Vatican secretary of state, Cardinal Pietro Parolin, told ANSA news agency that Leo had been "very calm" about his condition when they met on Monday.
The Vatican told AFP the nodule was found during routine health checks, and there were no symptoms that led to checking the pope's lungs.
Chicago-born Leo, the first US pope, is sporty and believed to generally be in good health.
He is scheduled to travel to South America from November 6-17 to visit Uruguay, Argentina and Peru.