A video circulated on social media shows young people handing the visibly delighted pontiff a pizza box in the Popemobile.
The"pizza messengers" had drawn attention to themselves with posters reading "We have Aurelio's Pizza" and "Pope Leo, I brought you pizza from Chicago".
The pizza was from a chain in Homewood, Illinois, which Robert Francis Prevost, Pope Leo XIV since 8 May, used to frequent.
The owner created his own recipe for Leo: "Poperoni" pizza - extra spicy for the 69-year-old, who has loved spicy food at least since his time as a priest and bishop in Peru.
The scene took place before the general audience with many participants in the World Youth Meeting, which the Vatican is expecting to attract at least half a million people by Sunday.