During a mass in Naples the 78-year-old was given a vial of dried blood of St. Gennaro, the Italian city's patron saint who died in the fourth century.
After the 266th pontiff kissed the sealed glass ampoule it incredibly began to turn into liquid.
Cardinal Crescenzo Sepe declared to a cheering crowd that watched the moment that Pope Francis had performed a miracle.
He said: "It is the sign that St. Gennaro loves Pope Francis: half of the blood turned to liquid."
The Argentine leader of the Catholic Church seemed surprise by the cardinal's announcement and took hold of the microphone afterwards to make a joke.
The pontiff said: "The bishop said the blood is only half liquefied. It appears the saint only loves us half-way.
"We have to convert ourselves (to good) more so he loves us more."
Catholics believe the "miracle" of St. Gennaro can occur three times a year on certain regular feast days.
However, they feel this will only occur if they pray enough after it is taken out of a vault and put on display.
The ritual of the liquefying blood dates back to the 14th century.
The last time the blood turned into liquid in the papal presence was 1848 when it did so in front of Pope Pius IX.
Sceptics believe the "miracle" is due to chemicals present in the ampoule that make the viscosity of the blood change when it is moved.
This happened on Saturday when it was removed from the vault for the pope's visit.
By Dion Dassanayake, EXPRESS