Markit/ADACI's services purchasing managers' index (PMI) for Italy dropped to 51.2 in March from 53.8 in February. It was the lowest level since March 2015.
A reading above 50 indicates expansion in the sector.
The index was mainly driven by a softer growth in new business.
"The drop in the services PMI signals a loss of growth momentum at the end of the first quarter," an economist at Markit Phil Smith said.