The monthly figure marks an increase from 12.0 percent in April, bringing Italy's jobless rate to 3.1 million people.
The unemployment rate is higher for women, Istat's provisional estimates showed, with 13.2 percent of women out of work compared to 11.5 percent of men.
The news follows an EU summit on youth unemployment last week, during which Italian Prime Minister Enrico Letta secured a €1.5 billion package to get young people into work. Youth unemployment in Italy stands at 40 percent, compared to the EU average of 23.5 percent.
Letta's jobs package will initially be used in the south of Italy, where the problem is particularly acute.