Pope Leo says doctors have detected a small growth on his lung during a recent health check and he will have this week to remove it.
It is the first health scare of the 71-year-old pontiff's tenure.
Leo told pilgrims in St Peter's Square for his weekly Sunday prayer that the growth, a small nodule, was "well-contained" and had been spotted early.
The Vatican did not say whether the nodule is believed to be malignant.
"Recently, during some medical examinations, doctors found a localised nodule in my lung," Leo said.
"It was detected at a very early stage and is well contained.
"The doctors believe it is advisable to intervene, so, following their advice, I have decided to undergo surgery to remove it.
"The operation is already scheduled for the next few days."
A nodule on the lung is a small, localised lesion that can be benign. The American Thoracic Society says fewer than 5% of lung nodules overall prove cancerous, although risk rises with age, size and other factors such as smoking.
"Lung nodules are fairly common and are often benign," said Stephen Liu, a lung cancer expert at Georgetown University in Washington and division chief of hematology-oncology at MedStar Georgetown University Hospital.
"When there is sufficient concern, and when it is safe to do so, based on the location of the nodule and the overall health of the patient, surgical removal can provide a definitive diagnosis.”
RELATIVELY YOUNG POPE, PLAYS SPORTS
Leo, from the United States and elected pope by the world's cardinals in May 2025, is relatively young for a Church leader.
He enjoys sports, including tennis, and is not known to be a smoker or to previously have had health issues.
Italian Prime Minister Giorgia Meloni wished the Pope well.
"I would like to express to the Holy Father my personal support, and that of the Italian government, as he faces this challenge in the coming days," she said in a statement.
"The serenity and composure with which he spoke about it (today) … are reassuring," Meloni added.
The late Pope Francis, Leo's predecessor, died at age 88. He was hospitalised several times as he grew frail in his later years. He did not usually announce his hospital visits in advance, sometimes sparking a media frenzy over his health.
The Vatican press office said Leo's recovery from surgery was expected to take about a week. It did not immediately provide further details about the Pope's health.
Leo keeps a busy schedule and is due to make an 11-day tour of Uruguay, Argentina and Peru from November 6-17.
Before mentioning his health issue in his address on Sunday, the Pope referred to the trip, thanking Catholics for their prayers "for the pastoral visit I will undertake."