The 92-year-old had suffered a ruptured aorta, the largest artery in the body.
"The surgery was a success, it was a complex intervention," said Francesco Musumeci who runs the cardiac surgery unit at the Saint Camillo hospital in Rome.
"The heart has resumed its function, now we need to wait."
In office between 2006 and 2015, Napolitano was considered a stalwart of stability during a particularly turbulent period in Italy – from the truncated premiership of Romano Prodi to the curtailed mandates of Silvio Berlusconi, Mario Monti and Enrico Letta.
The country also experienced its gravest economic recession since the post-war period.
In 2013, Napolitano agreed to serve an unprecedented second term amid a fierce political deadlock, but resigned two years later because of his advancing age.
Photo: Tiziana Fabi/AFP
Born in Naples on June 29, 1925 into a family of intellectuals, Napolitano took part in the resistance against Nazi and fascist troops during World War II, founding a communist group.
At the end of the war, he joined the Italian Communist Party and was elected to parliament in 1953 after earning a law degree.
Napolitano was one of the most influential leaders of the party's reformist wing, although he notoriously supported the Soviet Union's invasion of Hungary in 1956 to crush a liberal revolution.
With the collapse of the USSR, the Italian Communist Party was officially disbanded in 1991. The current Democratic Party is its main heir.
After turns as lower house speaker, interior minister and leftwing MEP, he became Italy's first ex-communist to be elected president in 2006.
The veteran held the rare quality of being respected by both right and left and an ability to stay above the party political fray.
Earlier this month, he assisted talks with President Sergio Mattarella who seeks to break the current government deadlock after last month's election left a hung parliament.