The woman, reportedly a psychology trainee, was monitored overnight in hospital in the Tuscan city of Massa after a nurse accidentally gave her six doses of the vaccine on Sunday.
She was said to be in good condition after receiving fluids and paracetamol, Italian news agency AGI reported.
The woman was released later on Monday.
Instead of just one dose, a nurse mistakenly injected the entire vial, the equivalent of six doses.
Previous studies to test overdoses of the Pfizer vaccine were limited to four doses, AGI wrote.
Overdoses of the Pfizer vaccine have previously been reported in the United States, Australia, Germany and Israel.
The incident has been reported to Italy’s medicines regulator.
As of Monday, there had been over 24 million COVID-19 vaccine doses administered in Italy.
So far, just over 7.4 million people in Italy can be said to be fully protected against COVID-19.
