Markit/ADACI's services purchasing managers' index (PMI) for Italy dropped to 53.6 in January from 55.3 in December.
A reading above 50 indicates expansion in the sector.
The index was driven by a slower growth in new business and job creation.
"The service sector began the year growing at a softer pace than was seen in December; however this shouldn't raise too many eyebrows as December marked one of the best months in quite some time. It was encouraging to see new business rise again after a marked gain in the month before, which maintains momentum and should mean that more jobs are added in the coming months," an economist at Markit Phil Smith said.