Markit/ADACI's services purchasing managers' index (PMI) for Italy was down to 52.0 in July from 53.4 in June.
A reading above 50 indicates expansion in the sector.
The fall was driven by a decline in employment.
"The service sector maintained its recent growth spell at the start of the third quarter and has now expanded in each of the past five months - the longest sequence since 2010. However, rather than building momentum, as is the case in manufacturing, growth appears to be fixed in a low gear," an economist at Markit Phil Smith said.