The rate among 15 to 24-year-olds climbed to a record 43.7 percent from 43.1 percent in May, data from the national statistics institute (Istat) showed on Thursday.
However, unemployment in the adult population defied analysts' expectations by easing to 12.3 percent, from the 12.6 percent recorded in both April and May.
The total number of employed grew by 50,000 persons on a month, with more women than men finding work, Istat said.
"The job market is stabilizing, though a significant drop in unemployment rates is not expected until 2015," Intesa San Paolo bank said in a note.
Renzi has promised his "jobs act" will help young people in particular as Italy attempts to pull out of the deepest recession since the Second World War.
Both the Bank of Italy and International Monetary Fund (IMF) this month slashed their 2014 growth forecasts to 0.2 percent and 0.3 percent respectively.