The jobless rate rose from 12.5 percent in August to 12.6 percent in September, data from the national statistics institute (Istat) showed on Friday.
Economists had tabled on a drop to 12.4 percent.
According data adjusted for seasonal blips, 3.24 million people were actively seeking work, up 1.5 percent on the figure a month earlier and 1.8 percent on a year.
Youth unemployment, which had been at a record high, dipped slightly to 42.9 percent from 43.7 percent in September.
Renzi has promised his "Jobs Act" will reboot Italy's economy and lure in foreign investors, but he has clashed with the unions over plans to make it easier for companies to fire people.
Protesters from across Italy descended on Rome last weekend for a vast rally against the measures, and the CGIL – the largest union and a historic ally of Renzi's centre-left Democratic Party – has attacked the government and announced a general strike.
Italy narrowly escaped its third recession in the last six years this month but the country still has forecast a debt-to-GDP ratio of 133.4 percent in 2015 – more than twice the EU ceiling.