Markit/ADACI's services purchasing managers' index (PMI) for Italy decreased to 53.3 in September from 54.6 in August.
A reading above 50 indicates expansion in the sector.
The decline was driven by a weaker rise in new business and by a drop in employment.
"In both manufacturing and services new business growth eased once more, to the weakest in seven months, while backlogs of work posted renewed contractions. These give some warnings about growth prospects in the final few months of the year," an economist at Markit Phil Smith said.