The 71-year-old Leo said the nodule was found at an early stage and was "circumscribed" or small and well-localised. The Vatican said Leo's expected recovery time from surgery would be a week.
The American Pope made the announcement during his Sunday noon blessing at St Peter's Square in an unusual display of transparency about his health.
"Thank you for your spiritual support, and I entrust myself to your prayers," he said.
Vatican News said the operation would take place at Gemelli Hospital in Rome, where popes before him were treated and where there is a special apartment for pontiffs.
Leo has one upcoming main event: the closing Mass on Wednesday of a week-long conference on family ministry, but Vatican officials suggested he might miss it.
A lung nodule is a small abnormal area measuring less than three centimetres that sometimes appears on a CT scan of the chest or abdomen, according to the American Cancer Society.
Often, the scans are done to investigate trauma or a lung infection, unrelated to a lung cancer screening.
"Lung nodules are fairly common and are often benign, but benign and cancerous nodules can have a similar appearance on imaging," said Stephen Liu, a lung cancer expert at Georgetown's Lombardi Comprehensive Cancer Center.
"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," Liu said.
If the nodule turns out to be cancerous, the prognosis is "excellent" since it will have been removed at an early stage and surgery alone might be all Leo needs in terms of treatment, he said.
"It often can be a blessing in disguise to remove that cancer before it has a chance to grow or to spread," he said.
Leo has no previously known health problems and is physically active, with weekly tennis matches and swimming at the papal summer retreat in Castel Gandolfo.
The Pope has a major trip planned November 6-17 to Uruguay, Argentina and Peru.
It's unclear how or whether the planned surgery and recovery might affect the itinerary, but Leo said he still planned to go.
In his remarks, delivered in Italian, Leo said that during a recent medical check, "doctors found a circumscribed nodule in my lung".
"It was detected at a very early stage and is well-localised. The doctors believe it's best to take action, so, following their advice, I have decided to have surgery to remove it."
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